diff --git a/google/downscope/downscoping.go b/google/downscope/downscoping.go
index 1995428..c1a6a8f 100644
--- a/google/downscope/downscoping.go
+++ b/google/downscope/downscoping.go
@@ -22,7 +22,7 @@ import (
 	"golang.org/x/oauth2"
 )
 
-const (
+var (
 	identityBindingEndpoint = "https://sts.googleapis.com/v1/token"
 )
 
@@ -96,19 +96,22 @@ func NewTokenSource(ctx context.Context, conf DownscopingConfig) downscopingToke
 	return downscopingTokenSource{ctx: ctx, config: conf}
 }
 
-// downscopedTokenWithEndpoint is a helper function used for unit testing
-// purposes, as it allows us to pass in a locally mocked endpoint.
-func downscopedTokenWithEndpoint(ctx context.Context, config DownscopingConfig, endpoint string) (*oauth2.Token, error) {
-	if config.RootSource == nil {
+// Token() uses a downscopingTokenSource to generate an oauth2 Token.
+// Do note that the returned TokenSource is an oauth2.StaticTokenSource. If you wish
+// to refresh this token automatically, then initialize a locally defined
+// TokenSource struct with the Token held by the StaticTokenSource and wrap
+// that TokenSource in an oauth2.ReuseTokenSource.
+func (dts downscopingTokenSource) Token() (*oauth2.Token, error) {
+	if dts.config.RootSource == nil {
 		return nil, fmt.Errorf("downscope: rootTokenSource cannot be nil")
 	}
-	if len(config.Rules) == 0 {
+	if len(dts.config.Rules) == 0 {
 		return nil, fmt.Errorf("downscope: length of AccessBoundaryRules must be at least 1")
 	}
-	if len(config.Rules) > 10 {
+	if len(dts.config.Rules) > 10 {
 		return nil, fmt.Errorf("downscope: length of AccessBoundaryRules may not be greater than 10")
 	}
-	for _, val := range config.Rules {
+	for _, val := range dts.config.Rules {
 		if val.AvailableResource == "" {
 			return nil, fmt.Errorf("downscope: all rules must have a nonempty AvailableResource: %+v", val)
 		}
@@ -121,11 +124,11 @@ func downscopedTokenWithEndpoint(ctx context.Context, config DownscopingConfig,
 		Boundary accessBoundary `json:"accessBoundary"`
 	}{
 		Boundary: accessBoundary{
-			AccessBoundaryRules: config.Rules,
+			AccessBoundaryRules: dts.config.Rules,
 		},
 	}
 
-	tok, err := config.RootSource.Token()
+	tok, err := dts.config.RootSource.Token()
 	if err != nil {
 		return nil, fmt.Errorf("downscope: unable to obtain root token: %v", err)
 	}
@@ -142,8 +145,8 @@ func downscopedTokenWithEndpoint(ctx context.Context, config DownscopingConfig,
 	form.Add("subject_token", tok.AccessToken)
 	form.Add("options", string(b))
 
-	myClient := oauth2.NewClient(ctx, nil)
-	resp, err := myClient.PostForm(endpoint, form)
+	myClient := oauth2.NewClient(dts.ctx, nil)
+	resp, err := myClient.PostForm(identityBindingEndpoint, form)
 	if err != nil {
 		return nil, fmt.Errorf("unable to generate POST Request %v", err)
 	}
@@ -180,12 +183,3 @@ func downscopedTokenWithEndpoint(ctx context.Context, config DownscopingConfig,
 	}
 	return newToken, nil
 }
-
-// Token() uses a downscopingTokenSource to generate an oauth2 Token.
-// Do note that the returned TokenSource is an oauth2.StaticTokenSource. If you wish
-// to refresh this token automatically, then initialize a locally defined
-// TokenSource struct with the Token held by the StaticTokenSource and wrap
-// that TokenSource in an oauth2.ReuseTokenSource.
-func (dts downscopingTokenSource) Token() (*oauth2.Token, error) {
-	return downscopedTokenWithEndpoint(dts.ctx, dts.config, identityBindingEndpoint)
-}
diff --git a/google/downscope/downscoping_test.go b/google/downscope/downscoping_test.go
index e79edc8..d5adda1 100644
--- a/google/downscope/downscoping_test.go
+++ b/google/downscope/downscoping_test.go
@@ -46,7 +46,9 @@ func Test_DownscopedTokenSource(t *testing.T) {
 	}
 	myTok := oauth2.Token{AccessToken: "Mellon"}
 	tmpSrc := oauth2.StaticTokenSource(&myTok)
-	_, err := downscopedTokenWithEndpoint(context.Background(), DownscopingConfig{tmpSrc, new}, ts.URL)
+	dts := downscopingTokenSource{context.Background(), DownscopingConfig{tmpSrc, new}}
+	identityBindingEndpoint = ts.URL
+	_, err := dts.Token()
 	if err != nil {
 		t.Fatalf("NewDownscopedTokenSource failed with error: %v", err)
 	}