WASHINGTON — The Supreme Court will take up a case from Alabama next week to decide whether to strike down a key part of the Voting Rights Act of 1965, a landmark measure that made voting a reality for blacks in the South and won extension by a near-unanimous vote from Congress in 2006.

Critics on the right agree the law was a success, but they contend it is now outdated and unfair to the South. They also say it is used mostly as a way to force states to draw electoral districts that favor black or Latino candidates.

But liberal legal scholars have urged the justices to step back and pay attention to the history of the Reconstruction era. They are pointedly addressing the conservatives, led by Justice Antonin Scalia, who say the court should follow the actual words and original understanding of the Constitution.

The 15th Amendment, added in 1870, says the right to vote “shall not be abridged or denied … on account of race” and “Congress shall have the power to enforce this article by appropriate legislation.”

Yale Law School professor Akhil Amar, an expert on how the Reconstruction era changed the Constitution, says the 15th Amendment made clear that Congress, not the courts and certainly not the Southern states, was entrusted with protecting voting rights. So the Voting Rights Act “is obviously appropriate legislation,” he said.

“This is as pure a test as you could have as to whether the conservatives will follow the text and history of the Constitution if it leads to a result they may not like,” said Doug Kendall, president of the Constitutional Accountability Center.

But civil rights lawyers are worried nonetheless. Four years ago, the high court sounded a warning that the days of the Voting Rights Act may be limited.

“Things have changed in the South,” said Chief Justice John G. Roberts Jr. He noted that blacks now register and vote at rates the same as or higher than whites in several Southern states, including Mississippi and Georgia. Justice Anthony M. Kennedy said he was troubled by a law that treats Georgia differently from Ohio.

He was referring to Section 5 of the law, the disputed provision that requires states and municipalities mainly in the South to seek clearance from Washington before they change their voting rules or election districts. “It deters mischief,” said Rep. Melvin Watt, a black Democrat from North Carolina.

In its early years, the Voting Rights Act prevented schemes that made it hard for blacks to register and cast ballots. But more recently, it has focused on what Congress called the more subtle “second generation” problems of ensuring that minority votes translate into political power.

On the local level, one suspect tactic has been to switch how members of a city council or county board are elected. If blacks were one-third of the voters and the members of a county board were elected by districts, the likelihood is that blacks could elect one or more of their favored candidates to a seven-member board. But if the members are elected countywide, a white majority could elect an all-white board.

The NAACP Legal Defense and Educational Fund says more than 180 cities, counties and school boards in Alabama were challenged in recent decades for using “at-large election systems tainted by racial discrimination.” These cases rarely get much attention, but they can determine whether black and Latino voters have much influence in local governments and state legislatures.

Last year, the Obama administration and Texas Republicans clashed in several high-profile disputes over voting rights. The GOP-controlled Legislature redrew congressional districts in a way that that favored Republican incumbents, but the administration said the new map undercut the voting power of the growing Latino population. A three-judge court in Washington blocked the plan as discriminatory, citing the Voting Rights Act. A separate three-judge court also stopped a Texas voter identification law from taking effect.

But since the 1990s, the Supreme Court’s conservative justices have voiced frustration with the Voting Rights Act and said it had become a tool for “racial gerrymandering.” In a series of rulings, they struck down odd-shaped districts that were drawn to help elect black or Latino candidates.

Abigail Thernstrom, vice chairwoman of the U.S. Commission on Civil Rights and a longtime critic of the law, said that despite its early success, “Section 5 has outlived its usefulness. It’s a relic from the era of the Jim Crow South.”

Four years ago, the court took up a broad challenge to the “pre-clearance” part of the Voting Rights Act in a case brought by a Texas water district, but it settled on a small ruling. The water district, aptly enough, was told it could “bail out” from the law.

Only Justice Clarence Thomas said the law’s special coverage for the South was unconstitutional, but it was clear that several of his fellow conservatives agreed.

Three days after President Obama won reelection, the court announced it would hear a challenge to the Voting Rights Act from Shelby County, Ala., which contends the law “usurps the state’s sovereign powers” and should be declared unconstitutional. The arguments will be heard Feb. 27.

david.savage@latimes.com