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 boasted of their racial discrimination still can be forced in the 21st century to get federal permission before making changes in the way they hold elections.

Some of the governments covered — most of them are in the South — argue they have turned away from racial discrimination over the years. But Congress and lower courts that have looked at recent challenges to the law concluded that a history of discrimination and more recent efforts to harm minority voters justify continuing federal oversight.

The Supreme Court took no action Monday on cases asking it to end the Voting Rights Act's advance approval requirement that has been held up as a crown jewel of the civil rights era.

The justices sidestepped this very issue in a case from Texas in 2009. In an opinion joined by eight justices, Chief Justice John Roberts wrote then that the issue of advance approval "is a difficult constitutional question we do not answer today."

Since then, Congress has not addressed potential problems identified by the court. Meanwhile, the law's opponents sensed its vulnerability and filed several new lawsuits.