diff --git a/.coveragerc b/.coveragerc
index be6eccb..00ae33c 100644
--- a/.coveragerc
+++ b/.coveragerc
@@ -2,7 +2,6 @@
 branch = True
 
 [report]
-fail_under = 100
 show_missing = True
 omit =
     google/cloud/servicemanagement/__init__.py
diff --git a/google/cloud/servicemanagement_v1/services/service_manager/async_client.py b/google/cloud/servicemanagement_v1/services/service_manager/async_client.py
index 083495a..d2cf585 100644
--- a/google/cloud/servicemanagement_v1/services/service_manager/async_client.py
+++ b/google/cloud/servicemanagement_v1/services/service_manager/async_client.py
@@ -18,6 +18,7 @@
 import re
 from typing import Dict, Sequence, Tuple, Type, Union
 import pkg_resources
+import warnings
 
 import google.api_core.client_options as ClientOptions  # type: ignore
 from google.api_core import exceptions as core_exceptions  # type: ignore
@@ -1443,6 +1444,11 @@ async def enable_service(
                 Operation payload for EnableService method.
 
         """
+        warnings.warn(
+            "ServiceManagerAsyncClient.enable_service is deprecated",
+            DeprecationWarning,
+        )
+
         # Create or coerce a protobuf request object.
         # Sanity check: If we got a request object, we should *not* have
         # gotten any keyword arguments that map to the request.
@@ -1544,6 +1550,11 @@ async def disable_service(
                 Operation payload for DisableService method.
 
         """
+        warnings.warn(
+            "ServiceManagerAsyncClient.disable_service is deprecated",
+            DeprecationWarning,
+        )
+
         # Create or coerce a protobuf request object.
         # Sanity check: If we got a request object, we should *not* have
         # gotten any keyword arguments that map to the request.
diff --git a/google/cloud/servicemanagement_v1/services/service_manager/client.py b/google/cloud/servicemanagement_v1/services/service_manager/client.py
index 7c291ca..e3c0276 100644
--- a/google/cloud/servicemanagement_v1/services/service_manager/client.py
+++ b/google/cloud/servicemanagement_v1/services/service_manager/client.py
@@ -19,6 +19,7 @@
 import re
 from typing import Callable, Dict, Optional, Sequence, Tuple, Type, Union
 import pkg_resources
+import warnings
 
 from google.api_core import client_options as client_options_lib  # type: ignore
 from google.api_core import exceptions as core_exceptions  # type: ignore
@@ -1614,6 +1615,10 @@ def enable_service(
                 Operation payload for EnableService method.
 
         """
+        warnings.warn(
+            "ServiceManagerClient.enable_service is deprecated", DeprecationWarning,
+        )
+
         # Create or coerce a protobuf request object.
         # Sanity check: If we got a request object, we should *not* have
         # gotten any keyword arguments that map to the request.
@@ -1715,6 +1720,10 @@ def disable_service(
                 Operation payload for DisableService method.
 
         """
+        warnings.warn(
+            "ServiceManagerClient.disable_service is deprecated", DeprecationWarning,
+        )
+
         # Create or coerce a protobuf request object.
         # Sanity check: If we got a request object, we should *not* have
         # gotten any keyword arguments that map to the request.
diff --git a/google/cloud/servicemanagement_v1/services/service_manager/transports/base.py b/google/cloud/servicemanagement_v1/services/service_manager/transports/base.py
index 1414f04..e2b007e 100644
--- a/google/cloud/servicemanagement_v1/services/service_manager/transports/base.py
+++ b/google/cloud/servicemanagement_v1/services/service_manager/transports/base.py
@@ -25,6 +25,7 @@
 from google.api_core import retry as retries  # type: ignore
 from google.api_core import operations_v1  # type: ignore
 from google.auth import credentials as ga_credentials  # type: ignore
+from google.oauth2 import service_account  # type: ignore
 
 from google.api import service_pb2  # type: ignore
 from google.cloud.servicemanagement_v1.types import resources
@@ -49,8 +50,6 @@
     except pkg_resources.DistributionNotFound:  # pragma: NO COVER
         _GOOGLE_AUTH_VERSION = None
 
-_API_CORE_VERSION = google.api_core.__version__
-
 
 class ServiceManagerTransport(abc.ABC):
     """Abstract transport class for ServiceManager."""
@@ -73,6 +72,7 @@ def __init__(
         scopes: Optional[Sequence[str]] = None,
         quota_project_id: Optional[str] = None,
         client_info: gapic_v1.client_info.ClientInfo = DEFAULT_CLIENT_INFO,
+        always_use_jwt_access: Optional[bool] = False,
         **kwargs,
     ) -> None:
         """Instantiate the transport.
@@ -96,6 +96,8 @@ def __init__(
                 API requests. If ``None``, then default info will be used.
                 Generally, you only need to set this if you're developing
                 your own client library.
+            always_use_jwt_access (Optional[bool]): Whether self signed JWT should
+                be used for service account credentials.
         """
         # Save the hostname. Default to port 443 (HTTPS) if none is specified.
         if ":" not in host:
@@ -105,7 +107,7 @@ def __init__(
         scopes_kwargs = self._get_scopes_kwargs(self._host, scopes)
 
         # Save the scopes.
-        self._scopes = scopes or self.AUTH_SCOPES
+        self._scopes = scopes
 
         # If no credentials are provided, then determine the appropriate
         # defaults.
@@ -124,13 +126,20 @@ def __init__(
                 **scopes_kwargs, quota_project_id=quota_project_id
             )
 
+        # If the credentials is service account credentials, then always try to use self signed JWT.
+        if (
+            always_use_jwt_access
+            and isinstance(credentials, service_account.Credentials)
+            and hasattr(service_account.Credentials, "with_always_use_jwt_access")
+        ):
+            credentials = credentials.with_always_use_jwt_access(True)
+
         # Save the credentials.
         self._credentials = credentials
 
-    # TODO(busunkim): These two class methods are in the base transport
+    # TODO(busunkim): This method is in the base transport
     # to avoid duplicating code across the transport classes. These functions
-    # should be deleted once the minimum required versions of google-api-core
-    # and google-auth are increased.
+    # should be deleted once the minimum required versions of google-auth is increased.
 
     # TODO: Remove this function once google-auth >= 1.25.0 is required
     @classmethod
@@ -151,27 +160,6 @@ def _get_scopes_kwargs(
 
         return scopes_kwargs
 
-    # TODO: Remove this function once google-api-core >= 1.26.0 is required
-    @classmethod
-    def _get_self_signed_jwt_kwargs(
-        cls, host: str, scopes: Optional[Sequence[str]]
-    ) -> Dict[str, Union[Optional[Sequence[str]], str]]:
-        """Returns kwargs to pass to grpc_helpers.create_channel depending on the google-api-core version"""
-
-        self_signed_jwt_kwargs: Dict[str, Union[Optional[Sequence[str]], str]] = {}
-
-        if _API_CORE_VERSION and (
-            packaging.version.parse(_API_CORE_VERSION)
-            >= packaging.version.parse("1.26.0")
-        ):
-            self_signed_jwt_kwargs["default_scopes"] = cls.AUTH_SCOPES
-            self_signed_jwt_kwargs["scopes"] = scopes
-            self_signed_jwt_kwargs["default_host"] = cls.DEFAULT_HOST
-        else:
-            self_signed_jwt_kwargs["scopes"] = scopes or cls.AUTH_SCOPES
-
-        return self_signed_jwt_kwargs
-
     def _prep_wrapped_messages(self, client_info):
         # Precompute the wrapped methods.
         self._wrapped_methods = {
diff --git a/google/cloud/servicemanagement_v1/services/service_manager/transports/grpc.py b/google/cloud/servicemanagement_v1/services/service_manager/transports/grpc.py
index bfee8dc..66f10bd 100644
--- a/google/cloud/servicemanagement_v1/services/service_manager/transports/grpc.py
+++ b/google/cloud/servicemanagement_v1/services/service_manager/transports/grpc.py
@@ -62,6 +62,7 @@ def __init__(
         client_cert_source_for_mtls: Callable[[], Tuple[bytes, bytes]] = None,
         quota_project_id: Optional[str] = None,
         client_info: gapic_v1.client_info.ClientInfo = DEFAULT_CLIENT_INFO,
+        always_use_jwt_access: Optional[bool] = False,
     ) -> None:
         """Instantiate the transport.
 
@@ -102,6 +103,8 @@ def __init__(
                 API requests. If ``None``, then default info will be used.
                 Generally, you only need to set this if you're developing
                 your own client library.
+            always_use_jwt_access (Optional[bool]): Whether self signed JWT should
+                be used for service account credentials.
 
         Raises:
           google.auth.exceptions.MutualTLSChannelError: If mutual TLS transport
@@ -155,6 +158,7 @@ def __init__(
             scopes=scopes,
             quota_project_id=quota_project_id,
             client_info=client_info,
+            always_use_jwt_access=always_use_jwt_access,
         )
 
         if not self._grpc_channel:
@@ -210,14 +214,14 @@ def create_channel(
               and ``credentials_file`` are passed.
         """
 
-        self_signed_jwt_kwargs = cls._get_self_signed_jwt_kwargs(host, scopes)
-
         return grpc_helpers.create_channel(
             host,
             credentials=credentials,
             credentials_file=credentials_file,
             quota_project_id=quota_project_id,
-            **self_signed_jwt_kwargs,
+            default_scopes=cls.AUTH_SCOPES,
+            scopes=scopes,
+            default_host=cls.DEFAULT_HOST,
             **kwargs,
         )
 
diff --git a/google/cloud/servicemanagement_v1/services/service_manager/transports/grpc_asyncio.py b/google/cloud/servicemanagement_v1/services/service_manager/transports/grpc_asyncio.py
index 3164b28..29b2258 100644
--- a/google/cloud/servicemanagement_v1/services/service_manager/transports/grpc_asyncio.py
+++ b/google/cloud/servicemanagement_v1/services/service_manager/transports/grpc_asyncio.py
@@ -83,14 +83,14 @@ def create_channel(
             aio.Channel: A gRPC AsyncIO channel object.
         """
 
-        self_signed_jwt_kwargs = cls._get_self_signed_jwt_kwargs(host, scopes)
-
         return grpc_helpers_async.create_channel(
             host,
             credentials=credentials,
             credentials_file=credentials_file,
             quota_project_id=quota_project_id,
-            **self_signed_jwt_kwargs,
+            default_scopes=cls.AUTH_SCOPES,
+            scopes=scopes,
+            default_host=cls.DEFAULT_HOST,
             **kwargs,
         )
 
@@ -108,6 +108,7 @@ def __init__(
         client_cert_source_for_mtls: Callable[[], Tuple[bytes, bytes]] = None,
         quota_project_id=None,
         client_info: gapic_v1.client_info.ClientInfo = DEFAULT_CLIENT_INFO,
+        always_use_jwt_access: Optional[bool] = False,
     ) -> None:
         """Instantiate the transport.
 
@@ -149,6 +150,8 @@ def __init__(
                 API requests. If ``None``, then default info will be used.
                 Generally, you only need to set this if you're developing
                 your own client library.
+            always_use_jwt_access (Optional[bool]): Whether self signed JWT should
+                be used for service account credentials.
 
         Raises:
             google.auth.exceptions.MutualTlsChannelError: If mutual TLS transport
@@ -201,6 +204,7 @@ def __init__(
             scopes=scopes,
             quota_project_id=quota_project_id,
             client_info=client_info,
+            always_use_jwt_access=always_use_jwt_access,
         )
 
         if not self._grpc_channel:
diff --git a/owlbot.py b/owlbot.py
index 99199e5..a273821 100644
--- a/owlbot.py
+++ b/owlbot.py
@@ -27,6 +27,15 @@
 for library in s.get_staging_dirs(default_version):
     s.move(library, excludes=["setup.py", "README.rst", "docs/index.rst"])
 
+    # Fix DeprecationWarning
+    # Fix incorrect DeprecationWarning
+    # Fixed in https://github.com/googleapis/gapic-generator-python/pull/943
+    s.replace(
+        "google/**/*client.py",
+        "warnings\.DeprecationWarning",
+        "DeprecationWarning"
+    )
+
 s.remove_staging_dirs()
 
 # ----------------------------------------------------------------------------
@@ -41,4 +50,4 @@
   templated_files, excludes=excludes
 )
 
-s.shell.run(["nox", "-s", "blacken"], hide_output=False)
\ No newline at end of file
+s.shell.run(["nox", "-s", "blacken"], hide_output=False)
diff --git a/setup.py b/setup.py
index 80c91d0..fefa6e4 100644
--- a/setup.py
+++ b/setup.py
@@ -25,7 +25,7 @@
 release_status = "Development Status :: 5 - Production/Stable"
 url = "https://github.com/googleapis/python-service-management"
 dependencies = [
-    "google-api-core[grpc] >= 1.22.2, < 2.0.0dev",
+    "google-api-core[grpc] >= 1.26.0, <2.0.0dev",
     "libcst >= 0.2.5",
     "proto-plus >= 1.15.0",
     "packaging >= 14.3",
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index 0b5003a..ce8cfca 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -19,7 +19,7 @@
 #
 # e.g., if setup.py has "foo >= 1.14.0, < 2.0.0dev",
 # Then this file should have foo==1.14.0
-google-api-core==1.22.2
+google-api-core==1.26.0
 libcst==0.2.5
 proto-plus==1.15.0
 packaging==14.3
diff --git a/tests/unit/gapic/servicemanagement_v1/test_service_manager.py b/tests/unit/gapic/servicemanagement_v1/test_service_manager.py
index 470a813..72e6ed7 100644
--- a/tests/unit/gapic/servicemanagement_v1/test_service_manager.py
+++ b/tests/unit/gapic/servicemanagement_v1/test_service_manager.py
@@ -62,9 +62,6 @@
 )
 from google.cloud.servicemanagement_v1.services.service_manager import pagers
 from google.cloud.servicemanagement_v1.services.service_manager import transports
-from google.cloud.servicemanagement_v1.services.service_manager.transports.base import (
-    _API_CORE_VERSION,
-)
 from google.cloud.servicemanagement_v1.services.service_manager.transports.base import (
     _GOOGLE_AUTH_VERSION,
 )
@@ -82,8 +79,9 @@
 import google.auth
 
 
-# TODO(busunkim): Once google-api-core >= 1.26.0 is required:
-# - Delete all the api-core and auth "less than" test cases
+# TODO(busunkim): Once google-auth >= 1.25.0 is required transitively
+# through google-api-core:
+# - Delete the auth "less than" test cases
 # - Delete these pytest markers (Make the "greater than or equal to" tests the default).
 requires_google_auth_lt_1_25_0 = pytest.mark.skipif(
     packaging.version.parse(_GOOGLE_AUTH_VERSION) >= packaging.version.parse("1.25.0"),
@@ -94,16 +92,6 @@
     reason="This test requires google-auth >= 1.25.0",
 )
 
-requires_api_core_lt_1_26_0 = pytest.mark.skipif(
-    packaging.version.parse(_API_CORE_VERSION) >= packaging.version.parse("1.26.0"),
-    reason="This test requires google-api-core < 1.26.0",
-)
-
-requires_api_core_gte_1_26_0 = pytest.mark.skipif(
-    packaging.version.parse(_API_CORE_VERSION) < packaging.version.parse("1.26.0"),
-    reason="This test requires google-api-core >= 1.26.0",
-)
-
 
 def client_cert_source_callback():
     return b"cert bytes", b"key bytes"
@@ -166,6 +154,36 @@ def test_service_manager_client_from_service_account_info(client_class):
         assert client.transport._host == "servicemanagement.googleapis.com:443"
 
 
+@pytest.mark.parametrize(
+    "client_class", [ServiceManagerClient, ServiceManagerAsyncClient,]
+)
+def test_service_manager_client_service_account_always_use_jwt(client_class):
+    with mock.patch.object(
+        service_account.Credentials, "with_always_use_jwt_access", create=True
+    ) as use_jwt:
+        creds = service_account.Credentials(None, None, None)
+        client = client_class(credentials=creds)
+        use_jwt.assert_not_called()
+
+
+@pytest.mark.parametrize(
+    "transport_class,transport_name",
+    [
+        (transports.ServiceManagerGrpcTransport, "grpc"),
+        (transports.ServiceManagerGrpcAsyncIOTransport, "grpc_asyncio"),
+    ],
+)
+def test_service_manager_client_service_account_always_use_jwt_true(
+    transport_class, transport_name
+):
+    with mock.patch.object(
+        service_account.Credentials, "with_always_use_jwt_access", create=True
+    ) as use_jwt:
+        creds = service_account.Credentials(None, None, None)
+        transport = transport_class(credentials=creds, always_use_jwt_access=True)
+        use_jwt.assert_called_once_with(True)
+
+
 @pytest.mark.parametrize(
     "client_class", [ServiceManagerClient, ServiceManagerAsyncClient,]
 )
@@ -3737,7 +3755,6 @@ def test_service_manager_transport_auth_adc_old_google_auth(transport_class):
         (transports.ServiceManagerGrpcAsyncIOTransport, grpc_helpers_async),
     ],
 )
-@requires_api_core_gte_1_26_0
 def test_service_manager_transport_create_channel(transport_class, grpc_helpers):
     # If credentials and host are not provided, the transport class should use
     # ADC credentials.
@@ -3771,84 +3788,6 @@ def test_service_manager_transport_create_channel(transport_class, grpc_helpers)
         )
 
 
-@pytest.mark.parametrize(
-    "transport_class,grpc_helpers",
-    [
-        (transports.ServiceManagerGrpcTransport, grpc_helpers),
-        (transports.ServiceManagerGrpcAsyncIOTransport, grpc_helpers_async),
-    ],
-)
-@requires_api_core_lt_1_26_0
-def test_service_manager_transport_create_channel_old_api_core(
-    transport_class, grpc_helpers
-):
-    # If credentials and host are not provided, the transport class should use
-    # ADC credentials.
-    with mock.patch.object(
-        google.auth, "default", autospec=True
-    ) as adc, mock.patch.object(
-        grpc_helpers, "create_channel", autospec=True
-    ) as create_channel:
-        creds = ga_credentials.AnonymousCredentials()
-        adc.return_value = (creds, None)
-        transport_class(quota_project_id="octopus")
-
-        create_channel.assert_called_with(
-            "servicemanagement.googleapis.com:443",
-            credentials=creds,
-            credentials_file=None,
-            quota_project_id="octopus",
-            scopes=(
-                "https://www.googleapis.com/auth/cloud-platform",
-                "https://www.googleapis.com/auth/cloud-platform.read-only",
-                "https://www.googleapis.com/auth/service.management",
-                "https://www.googleapis.com/auth/service.management.readonly",
-            ),
-            ssl_credentials=None,
-            options=[
-                ("grpc.max_send_message_length", -1),
-                ("grpc.max_receive_message_length", -1),
-            ],
-        )
-
-
-@pytest.mark.parametrize(
-    "transport_class,grpc_helpers",
-    [
-        (transports.ServiceManagerGrpcTransport, grpc_helpers),
-        (transports.ServiceManagerGrpcAsyncIOTransport, grpc_helpers_async),
-    ],
-)
-@requires_api_core_lt_1_26_0
-def test_service_manager_transport_create_channel_user_scopes(
-    transport_class, grpc_helpers
-):
-    # If credentials and host are not provided, the transport class should use
-    # ADC credentials.
-    with mock.patch.object(
-        google.auth, "default", autospec=True
-    ) as adc, mock.patch.object(
-        grpc_helpers, "create_channel", autospec=True
-    ) as create_channel:
-        creds = ga_credentials.AnonymousCredentials()
-        adc.return_value = (creds, None)
-
-        transport_class(quota_project_id="octopus", scopes=["1", "2"])
-
-        create_channel.assert_called_with(
-            "servicemanagement.googleapis.com:443",
-            credentials=creds,
-            credentials_file=None,
-            quota_project_id="octopus",
-            scopes=["1", "2"],
-            ssl_credentials=None,
-            options=[
-                ("grpc.max_send_message_length", -1),
-                ("grpc.max_receive_message_length", -1),
-            ],
-        )
-
-
 @pytest.mark.parametrize(
     "transport_class",
     [
@@ -3871,12 +3810,7 @@ def test_service_manager_grpc_transport_client_cert_source_for_mtls(transport_cl
             "squid.clam.whelk:443",
             credentials=cred,
             credentials_file=None,
-            scopes=(
-                "https://www.googleapis.com/auth/cloud-platform",
-                "https://www.googleapis.com/auth/cloud-platform.read-only",
-                "https://www.googleapis.com/auth/service.management",
-                "https://www.googleapis.com/auth/service.management.readonly",
-            ),
+            scopes=None,
             ssl_credentials=mock_ssl_channel_creds,
             quota_project_id=None,
             options=[
@@ -3985,12 +3919,7 @@ def test_service_manager_transport_channel_mtls_with_client_cert_source(
                 "mtls.squid.clam.whelk:443",
                 credentials=cred,
                 credentials_file=None,
-                scopes=(
-                    "https://www.googleapis.com/auth/cloud-platform",
-                    "https://www.googleapis.com/auth/cloud-platform.read-only",
-                    "https://www.googleapis.com/auth/service.management",
-                    "https://www.googleapis.com/auth/service.management.readonly",
-                ),
+                scopes=None,
                 ssl_credentials=mock_ssl_cred,
                 quota_project_id=None,
                 options=[
@@ -4037,12 +3966,7 @@ def test_service_manager_transport_channel_mtls_with_adc(transport_class):
                 "mtls.squid.clam.whelk:443",
                 credentials=mock_cred,
                 credentials_file=None,
-                scopes=(
-                    "https://www.googleapis.com/auth/cloud-platform",
-                    "https://www.googleapis.com/auth/cloud-platform.read-only",
-                    "https://www.googleapis.com/auth/service.management",
-                    "https://www.googleapis.com/auth/service.management.readonly",
-                ),
+                scopes=None,
                 ssl_credentials=mock_ssl_cred,
                 quota_project_id=None,
                 options=[