The Supreme Court has ruled that a key part of the Voting Rights Act—the landmark civil-rights law protecting racial minorities from discrimination at the polls—is unconstitutional in its current form.

The 5-4 decision, announced Tuesday morning, invalidates—at least for now—Section 5, a crucial tool for fighting racial discrimination in voting, and comes at a time of rising concern over efforts to restrict access to the ballot box. It represents a victory for conservatives, and a blow to the voting rights of millions of non-white Americans.

“The Supreme Court has effectively gutted one of the nation’s most important and effective civil rights laws,” said Jon Greenbaum, chief counsel of the Lawyers Committee for Civil Rights Under Law, in a response typical of the reaction from voting-rights advocates. “Minority voters in places with a record of discrimination are now at greater risk of being disenfranchised than they have been in decades. Today’s decision is a blow to democracy.”

Calling himself “deeply disappointed,” President Obama urged lawmakers to strengthen the Voting Rights Act in response to the ruling, though prospects for such a move appear dim. “I am calling on Congress to pass legislation to ensure every American has equal access to the polls,” said Obama in a statement.

Meanwhile, conservatives celebrated the ruling. “It is a victim of its own success and is no longer needed,” said Hans Von Spakovsky, a fellow at the Heritage Foundation and a prominent advocate of voting restrictions, in a press release, referring to Section 5. “Despite claims to the contrary, Americans will continue to be protected from discrimination by the other permanent provisions of the Voting Rights Act.”

Section 5 of the Voting Rights Act requires that certain jurisdictions with a history of racial discrimination, including most southern states, submit any changes to their election systems to the U.S. Justice Department for “pre-clearance.” The Justice Department is empowered to block changes that could reduce minority voting power. In the decision announced Thursday, Shelby County v. Holder, the court ruled that the formula used by Congress to decide which jurisdictions are covered under the law, known as Section 4b, violates the Constitution.

“Our country has changed, and while any racial discrimination in voting is too much, Congress must ensure that the legislation it passes to remedy that problem speaks to current conditions,” wrote Chief Justice John Roberts for the majority.

“There is no doubt that these improvements are in large part because of the Voting Rights Act,” Roberts added. “The Act has proved immensely successful at redressing racial discrimination and integrating the voting process.”

The court’s four other conservatives, Antonin Scalia, Clarence Thomas, Samuel Alito, and Anthony Kennedy, joined Roberts in the majority. The four liberal justices dissented.

If Congress were to pass new legislation changing the formula, Section 5 could be revived. And voting-rights advocates inside and outside Congress told msnbc before the ruling that they’ve already begun discussions toward that goal. But given the partisan nature of the issue, and the current level of congressional gridlock, it figures to be a very heavy lift.

Still, supporters of voting rights in Congress vowed to get to work. “I intend to take immediate action to ensure that we will have a strong and reconstituted Voting Rights Act that protects against racial discrimination in voting,” said Senate Judiciary Chair Pat Leahy in a statement. House minority Leader Nancy Pelosi said Congress should “tak[e] the court’s decision as our cue for further action to strengthen this legislation.”

In a rare step, Justice Ginsburg read her dissent from the bench, noting the irony in Roberts’s claim that it was the effectiveness of the Voting Rights Act in stopping race discrimination that had doomed Section 5. ”In the Court’s view, the very success of Section 5 of the Voting Rights Act demands its dormancy.”

Lawyers for Shelby County, Ala. argued that the south has made progress on race relations since the 1960’s, meaning Section 5 in its current form is no longer needed, and violates the 14th Amendment’s guarantee of equal protection. But in 2006, Congress overwhelmingly reauthorized the Voting Rights Act, finding that voting discrimination remains a problem in covered areas. And as the Justice Department noted, in recent years the provision has been used to block a slew of election changes in covered areas—including strict voter ID laws passed in 2011 by Texas and South Carolina, a Texas statewide redistricting plan that was found to have intentionally discriminated against minorities, and cutbacks to early voting in several Florida counties. And North Carolina—much though not all of which is covered under Section 5—passed a voter ID bill in April, which had appeared likely to be challenged under Section 5. All those measures were pushed by Republicans, and opposed by Democrats.

In a sign of how the ruling already is emboldening conservatives, Texas Attorney General Greg Abbott said in a statement that his state’s strict voter ID law “will take effect immediately.”

Still, restrictions on voting have not been confined to areas covered under Section 5. Since 2010, numerous states including Ohio, Pennsylvania, and Wisconsin have taken steps to make voting more difficult. The courts have blocked some though not all of those changes from taking effect.

Intentional racial discrimination in voting is still prohibited under a different part of the Voting Rights Act, Section 2. But voting-rights advocates say Section 2 is a far less effective tool than Section 5, because it puts the burden of proof on victims of discrimination, and forces them to sue after the fact—by which time an election might already have been held. Section 5, by contrast, requires the jurisdiction to affirmatively show that minority voting power won’t be set back, before the change it seeks to make can go into effect.

“Section 5 is the great stop sign, the great protector,” Barbara Arnwine, the president of the Lawyers Committee for Civil Rights, said in February as the Supreme Court was preparing to hear the case. “There is no other tool in the Voting Rights Act that is the equivalent of Section 5.”

Conservatives had set their sights on Section 5 after Congress’s 2006 re-authorization. In a 2009 case brought by a Texas municipal district seeking to “bail out” from Section 5, the court avoided ruling on the provision’s constitutionality, but several of the conservative justices had appeared to encourage future challenges.