Split along ideological and partisan lines, the justices voted 5-4 to strip the government of its most potent tool to stop voting bias - the requirement in the Voting Rights Act that all or parts of 15 states with a history of discrimination in voting, mainly in the South, get Washington's approval before changing the way they hold elections.
Chief Justice John Roberts, writing for a majority of conservative, Republican-appointed justices, said the law's provision that determines which states are covered is unconstitutional because it relies on 40-year-old data and does not account for racial progress and other changes in U.S. society.
The decision effectively puts an end to the advance approval requirement that has been used to open up polling places to minority voters in the nearly half century since it was first enacted in 1965, unless Congress can come up with a new formula that Roberts said meets "current conditions" in the United States. That seems unlikely to happen any time soon.
President Barack Obama, the nation's first black chief executive, issued a statement saying he was "deeply disappointed" with the ruling and calling on Congress to update the law.
But in the South, Alabama Gov. Robert Bentley said that, while the requirement was necessary in the 1960s, that was no longer the case. He said, "We have long lived up to what happened then, and we have made sure it's not going to happen again."
The advance approval, or preclearance, requirement shifted the legal burden and required governments that were covered to demonstrate that their proposed election changes would not discriminate.
Going forward, the outcome alters the calculus of passing election-related legislation in the affected states and local jurisdictions. The threat of an objection from Washington has hung over such proposals for nearly a half century. Unless Congress acts, that deterrent now is gone.
That prospect has upset civil rights groups which especially worry that changes on the local level might not get the same scrutiny as the actions of state legislatures.
Tuesday's decision means that a host of state and local laws that have not received Justice Department approval or have not yet been submitted can take effect. Prominent among those are voter identification laws in Alabama and Mississippi.
Texas Attorney General Greg Abbott, a Republican, said his state's voter ID law, which a panel of federal judges blocked as discriminatory, also would be allowed to take effect.
Justice Ruth Bader Ginsburg, dissenting from the ruling along with the court's three other liberal, Democratic appointees, said there was no mistaking the court's action.
"Hubris is a fit word for today's demolition" of the law, she said.
Reaction to the ruling from elected officials generally divided along partisan lines.
Mississippi Lt. Gov. Tate Reeves, a Republican, said in a news release, "The practice of preclearance unfairly applied to certain states should be eliminated in recognition of the progress Mississippi has made over the past 48 years."
But Democratic Rep. Bennie Thompson, the only black lawmaker in Mississippi's congressional delegation, said the ruling "guts the most critical portion of the most important civil rights legislation of our time."
Alabama Gov. Bentley, a Republican, pointed to his state's legislature - 27 percent black, similar to Alabama's overall population - as a sign of the state's progress.
The court challenge came from Shelby County, Ala., a Birmingham suburb.
The prior approval requirement had applied to the states of Alabama, Alaska, Arizona, Georgia, Louisiana, Mississippi, South Carolina, Texas and Virginia. It also covered certain counties in California, Florida, New York, North Carolina and South Dakota, and some local jurisdictions in Michigan. Coverage was triggered by past discrimination not only against blacks, but also against American Indians, Asian-Americans, Alaska Natives and Hispanics.
Obama, whose historic election was a subtext in the court's consideration of the case, pledged that his administration would continue to fight discrimination in voting. "While today's decision is a setback, it doesn't represent the end of our efforts to end voting discrimination," the president said. "I am calling on Congress to pass legislation to ensure every American has equal access to the polls."
Congress essentially ignored the court's threat to upend the voting rights law in a similar case four years ago. Roberts said the "failure to act leaves us today with no choice."
Congressional Democrats said they are eager to make changes, but Republicans were largely noncommittal.
Sen. Charles Schumer, D-N.Y., said he expects Republicans to block efforts to revive the law, even though a Republican-led Congress overwhelmingly approved its latest renewal in 2006 and President George W. Bush signed it into law.
"As long as Republicans have a majority in the House and Democrats don't have 60 votes in the Senate, there will be no preclearance. It is confounding that after decades of progress on voting rights, which have become part of the American fabric, the Supreme Court would tear it asunder," Schumer said.
Attorney General Eric Holder said the Justice Department "will not hesitate to take swift enforcement action, using every legal tool that remains available to us, against any jurisdiction that seeks to take advantage of the Supreme Court's ruling by hindering eligible citizens' full and free exercise of the franchise."
Those federal tools include other permanent provisions of the Voting Rights Act that prohibit discrimination and apply nationwide. But they place the burden of proof on the government and can be used only one case at a time.
The Obama administration and civil rights groups said there is a continuing need for the federal law and pointed to the Justice Department's efforts to block voter ID laws in South Carolina and Texas last year, as well as a redistricting plan in Texas that a federal court found discriminated against the state's large and growing Hispanic population.
The justices all agreed that discrimination in voting still exists.
But Roberts said that the covered states have largely eradicated the problems that caused them to be included in the first place.
"The coverage formula that Congress reauthorized in 2006 ignores these developments, keeping the focus on decades-old data relevant to decades-old problems, rather than current data reflecting current needs," the chief justice said.
Ginsburg countered that Congress had found that the prior approval provision was necessary "to prevent a return to old ways."
Instead, "the court today terminates the remedy that proved to be best suited to block that discrimination," she said in a dissent that she read aloud in the packed courtroom.
Ginsburg said the law continues to be necessary to protect against what she called subtler, "second-generation" barriers to voting. She identified one such effort as the switch to at-large voting from a district-by-district approach in a city with a sizable black minority. The at-large system allows the majority to "control the election of each city council member, effectively eliminating the potency of the minority's votes," she said.
Justice Clarence Thomas was part of the majority, but wrote separately to say anew that he would have struck down the advance approval requirement itself.
Civil rights lawyers condemned the ruling.
"The Supreme Court has effectively gutted one of the nation's most important and effective civil rights laws. Minority voters in places with a record of discrimination are now at greater risk of being disenfranchised than they have been in decades," said Jon Greenbaum, chief counsel for the Lawyers' Committee for Civil Rights Under Law.
The decision comes five months after Obama started his second term in the White House, re-elected by a diverse coalition of voters.
The high court is in the midst of a broad re-examination of the ongoing necessity of laws and programs aimed at giving racial minorities access to major areas of American life from which they once were excluded. The justices issued a modest ruling Monday that preserved affirmative action in higher education and will take on cases dealing with anti-discrimination sections of a federal housing law and another affirmative action case from Michigan next term.
The Alabama county's lawsuit acknowledged that the measure's strong medicine was appropriate and necessary to counteract decades of state-sponsored discrimination in voting, despite the Fifteenth Amendment's guarantee of the vote for black Americans.
But it asked whether there was any end in sight for a provision that intrudes on states' rights to conduct elections and was considered an emergency response when first enacted in 1965.
The county noted that the 25-year extension approved in 2006 would keep some places under Washington's oversight until 2031. And, the county said, it seemed not to account for changes that include the elimination of racial disparity in voter registration and turnout or the existence of allegations of race-based discrimination in voting in areas of the country that are not subject to the provision.
Associated Press writers Emily Wagster Pettus in Jackson, Miss., and Bob Johnson in Montgomery, Ala. contributed to this report.