By Drew Singer

WASHINGTON (Reuters) - A federal court on Thursday blocked a Texas law that would require voters to show photo identification before casting ballots, saying the measure would likely curtail the ability of minorities to vote in the November 6 presidential election.

Evidence showed the law did the most harm to African Americans and Hispanics, who are more likely to live in poverty, Circuit Judge David Tatel wrote for a three-judge panel of the U.S. District Court in Washington.

Those without the underlying documents to obtain ID would have to purchase them, discouraging poor voters, he wrote. During the July trial, lawyers from the U.S. Department of Justice equated that purchase to an illegal poll tax.

Texas Attorney General Greg Abbott, a Republican, said he would appeal the decision directly to the U.S. Supreme Court.

"Today's decision is wrong on the law and improperly prevents Texas from implementing the same type of ballot integrity safeguards that are employed by Georgia and Indiana, and were upheld by the Supreme Court," he said.

Texas had the legal burden to prove its requirement did not harm minority voters. Judge Tatel's opinion called all the state's evidence "unpersuasive, invalid, or both."

The requirement was adopted in 2011 by the Republican-dominated Texas legislature. Voters would have had to present one of six forms of photo ID before casting their ballots.

1965 ACT AT STAKE

Supporters said the requirement would discourage fraud at the polls, but opponents called it a guise to suppress minority voters more likely to support Democratic candidates.

"Chalk up another victory for fraud," Texas Gov. Rick Perry, a Republican, said in a statement.

But Elise Boddie, director of litigation for the NAACP Legal Defense Fund, said: "The whole point of our democracy is to make sure that voters have the opportunity to vote, and that's what today's ruling does."

In March, the Obama administration blocked the requirement using the 1965 Voting Rights Act, a law designed to protect the voting rights of minorities, primarily blacks in Southern states.

Texas sued the U.S. government, arguing that it would be easy for a person without an ID to obtain one before the November 6 election. Texas lawyers said most voters already had IDs, and those who did not could get IDs for free if they had the underlying documents.

Since the 2010 election, in which Republicans gained control of many state and local governments, 11 U.S. states have passed voter ID laws, among other election rule changes.

The Obama administration has used the Voting Rights Act to counter the new wave of Republican measures that include voter ID requirements, redistricting maps and new ballot formats. Since the 2010 election, there have been more lawsuits under the Voting Rights Act than in the previous 45 years combined.

SUPREME COURT REVIEW

The challenges are building toward a possible U.S. Supreme Court review of the part of the Voting Rights Act that allows the federal government to block rule changes in certain states.

Two challenges reached the Supreme Court on July 20. If the court agrees to hear those cases during the term starting in October, it will weigh whether the key provision of the civil rights-era law is still necessary.

Any review of the Texas decision would focus only on the state's requirement itself and not the constitutionality of the Voting Rights Act.

With the presidential election little more than two months away, states are scrambling to try to implement voter ID laws that face challenges in state and federal courts.

Thursday's Texas decision came midway through a Voting Rights Act trial challenging a similar voter ID requirement in South Carolina. On Wednesday, a U.S. district judge lifted restrictions on third-party voter registration drives in Florida, previously put in place under a new state election law backed by the state's Republican governor.

Earlier this week, a different panel of the same Washington federal court blocked Texas' redistricting maps under the Voting Rights Act, saying they discriminated against blacks and Hispanics. Texas will likely use interim maps drawn up by a San Antonio federal court for the November elections.

Texas State Rep. Trey Martinez Fischer, head of the Mexican American Legislative Caucus, said rulings on the state's redistricting and voter ID laws were proof that the entire Voting Rights Act was still necessary.

"In the span of three days, we have had Texas' dirty laundry aired on a national stage. A history that we are not very proud of. A history that we have all claimed that we have overcome, and we have been told, in the year 2012, no longer exists," Fischer told Reuters.

The case is Texas v. Holder, U.S. District Court, District of Columbia, No. 12-cr-00128.

(Additional reporting by Nate Raymond and Jonathan Stempel in New York and Jim Forsyth in San Antonio, Texas; Editing by Howard Goller and Todd Eastham)