The City of Rome v. United States (1980) p. 24
The court held that Congress could prohibit electoral schemes with discriminatory effects even as the Court itself concluded on the same day that such schemes were not themselves violative of the 15th Amend.
Justice Marshall: the Act’s ban on electoral changes that are discriminatory in effect is an appropriate method of promoting the purposes of the 15th Amend…Congress could rationally have concluded that…it was proper to prohibit changes that have a discriminatory impact.

The court has continued to grapple with the reach of the congressional power under enforcement provisions of the Reconstruction Amendments in several other cases since Katzenbach v. Morgan.