WASHINGTON – In striking down a centerpiece of the Voting Rights Act, a divided Supreme Court on Tuesday handed Southern states and conservatives a key victory and also posed a steep challenge for Congress.

The court, in one of the term’s most highly anticipated edicts, ruled, 5-4, that part of the 1965 law must be updated to account for how times have changed since Congress first wrote the groundbreaking voting rights legislation.

The ruling essentially exempts nine states, as well as certain political jurisdictions in other states, from the necessity of getting prior Justice Department approval for changes that might have an impact on local elections.

President Obama said he was “deeply disappointed” by the decision and called on Congress “to pass legislation to ensure every American has equal access to the polls.”

“There is no denying that the conditions that originally justified these measures no longer characterize voting in the covered jurisdictions,” Chief Justice John G. Roberts Jr. wrote for the conservative majority.

The decision in the case brought by Shelby County, Ala., technically leaves in place the so-called preclearance requirements under the law.

They require prior Justice Department approval, under Section 5, of everything from buying new voting machines and closing polling places to requiring photo identification and shifting district boundaries, all tools that can be used to encourage or discourage minority voting.

Practically speaking, the decision also effectively pulls the plug on preclearance for the time being, by striking down a related section that sets the formula for determining which political jurisdictions must meet the preclearance requirements.

The justices said that this part of the law, known as Section 4, was unconstitutional. “Coverage today is based on decades-old data and eradicated practices,” Roberts wrote, noting that “voter registration and turnout data in the covered states have risen dramatically in the years since.”

In 1965, for instance, only 27 percent of African-American adults in Georgia were registered to vote, compared with 62 percent of white adults. By 2004, African-American voter registration in Georgia had jumped to 64 percent, exceeding white registration. Other Southern states have shown similar trends.

The court’s decision leaves up to Congress the job of updating the preclearance formula, a tough political task that some lawmakers acknowledge may be impossible.

There’s little chance the current Congress – divided along partisan lines and polarized to the point of dysfunction _ will muster a consensus to do so. Until the formula is updated, however, preclearance itself is up in the air.

Almost immediately after the decision, Texas Attorney General Greg Abbott declared that his state would put into place a controversial voter-identification law without Justice Department approval.

In the meantime, Attorney General Eric H. Holder Jr. said Tuesday that the Justice Department “will continue to carefully monitor” political jurisdictions for voting rights impediments and “will not hesitate to take swift enforcement action” when necessary.

Voting rights advocates say that requiring prior Justice Department approval for electoral changes is more effective than chasing after individual violations after the fact.

Nine states currently are covered in their entirety by the preclearance requirements: Alabama, Alaska, Arizona, Georgia, Louisiana, Mississippi, South Carolina, Texas and Virginia. Selected jurisdictions in an additional seven states – California, Florida, Michigan, New Hampshire, New York, North Carolina and South Dakota – also are covered. In New York, the affected areas are the New York City boroughs of Brooklyn, Queens and the Bronx.

The court’s majority reasoned that the different treatment “sharply departs” from the principle that all the states are treated equally.

“While one state waits months or years and expends funds to implement a validly enacted law, its neighbor can typically put the same law into effect immediately,” Roberts wrote. The court’s other conservative justices – Antonin Scalia, Clarence Thomas and Samuel A. Alito Jr. – joined the decision, as did a frequent swing vote, Justice Anthony M. Kennedy.

Underscoring the unhappiness of the more liberal dissenting Justices Stephen G. Breyer, Sonia Sotomayor and Elena Kagan, Justice Ruth Bader Ginsburg read the dissent from the bench. “Voting discrimination still exists; no one doubts that,” Ginsburg said, “but the court today terminates the remedy that proved to be best suited to block that discrimination.”

The law, though revised several times, still pegs preclearance coverage in part to voting turnout or registration in the 1964, 1968 and 1972 elections.

When Congress first passed the law in 1965, the preclearance measures were expected to last only five years. Instead, they’ve been renewed with ever-longer extensions. The most recent renewal of the Voting Rights Act, in 2006, extended the provisions for another 25 years. It’s unclear exactly what happens with the preclearance requirements in the absence of a coverage formula.

“I don’t think that we would be able to come up with a remedy that would satisfy this court,” said Rep. Mel Watt, D-N.C., a key author of the 2006 renewal.