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.
Some of the governments covered — most of them 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 could say as early as Monday whether it will consider ending 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 the issue in a case from Texas in 2009. In an opinion joined by eight justices, Chief Justice John Roberts wrote 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.
The advance approval, or preclearance requirement, was adopted in the Voting Rights Act in 1965 to give federal officials a potent tool to defeat persistent efforts to keep blacks from voting.
The provision was a huge success, and Congress periodically has renewed it over the years. The most recent occasion was in 2006, when a Republican-led Congress overwhelmingly approved and President George W. Bush signed a 25-year extension.
The requirement currently applies to the states of Alabama, Alaska, Arizona, Georgia, Louisiana, Mississippi, South Carolina, Texas, and Virginia. It also covers certain counties in California, Florida, New York, North Carolina, and South Dakota, and some local jurisdictions in Michigan and New Hampshire.
Before these locations can change their voting rules, they must get approval either from the Justice Department’s civil rights division or from the federal district court in Washington that the new rules would not discriminate.
Congress compiled a 15,000-page record and documented hundreds of instances of apparent voting discrimination in the states covered by the law dating to 1982, the last time it had been extended.
Among the incidents in the congressional record:
■ In 1998, Webster County, Ga., tried to reduce the black population in several school board districts after citizens elected a majority-black school board for the first time.
■ In 2001, Kilmichael, Miss., canceled an election when a large number of African-American candidates sought local office following 2000 census results that showed blacks had become the majority in the city.
■ In 2004, Waller County, Texas, sought to limit early voting near a historically black college and threatened to prosecute students for illegal voting after two black students said they would run for office.
But in 2009, Roberts indicated the court was troubled about the ongoing need for a law in the face of dramatically improved conditions, including increased minority voter registration and turnout rates. Roberts attributed part of the change to the law itself. “Past success alone, however, is not adequate justification to retain the preclearance requirements,’’ he said.
He also raised concern that the formula by which states are covered relies on data that is now 40 years old. By some measures, states covered by the law were outperforming some that were not.
Jurisdictions required to obtain preclearance were chosen based on whether they had a test restricting the opportunity to register or vote and whether they had a voter registration or turnout rate below 50 percent.
In the federal court of appeals in the District of Columbia, Circuit Judge Stephen Williams objected that the law specifies that these criteria are measured by what happened in elections several decades ago. But writing for a majority that upheld preclearance, Circuit Judge David Tatel said the question is not whether old data is being used, but whether it helps identify jurisdictions with the worst discrimination problems.
‘‘If it does, then even though the formula rests on decades-old factors, the statute is rational,’’ Tatel said.
Shelby County, Ala., a well-to-do, mostly white bedroom community near Birmingham, adopted Roberts’s arguments in its effort to have the voting rights provision declared unconstitutional, but lost in the lower courts. The county’s appeal is among those being weighed by the high court.