WASHINGTON — Three years ago, the Supreme Court warned there could be constitutional problems with a landmark civil rights law that has opened voting booths to millions of African-Americans. Now, opponents of a key part of the Voting Rights Act are asking the high court to finish off that provision.

The basic question is whether state and local governments that once engaged in racial discrimination still can be forced in the 21st century to get federal permission before making changes in the way they hold elections.