(Reuters) - A federal judge on Thursday struck down a Texas law requiring voters to show identification at polls, saying it placed an unconstitutional burden on voters and discriminated against minorities.

In a ruling that follows a two-week trial in Corpus Christi of a lawsuit challenging the law, U.S. District Judge Nelva Gonzales Ramos also found that it amounted to an unconstitutional poll tax.

“The court holds that SB 14 creates an unconstitutional burden on the right to vote, has an impermissible discriminatory effect against Hispanics and African-Americans and was imposed with an unconstitutional discriminatory purpose,” Ramos wrote in a 147-page ruling.

The case is part of a strategy by the Obama administration to challenge voting laws it says discriminate by race in order to counter a U.S. Supreme Court ruling in June that freed states from strict federal oversight.

“We are extremely heartened by the court’s decision, which affirms our position that the Texas voter identification law unfairly and unnecessarily restricts access to the franchise,” U.S. Attorney General Eric Holder said in a written statement following the ruling.

“Even after the Voting Rights Act was seriously eroded last year, we vowed to continue enforcing the remaining portions of that statute as aggressively as possible,” Holder said. “This ruling is an important vindication of those efforts.”

But Texas Attorney General Greg Abbot said the state would “immediately” appeal Ramos’s ruling to the U.S. Fifth Circuit Court of Appeals and hoped to avoid confusion ahead of the November elections.

“The U.S. Supreme Court has already ruled that voter ID laws are constitutional, so we are confident the Texas law will be upheld on appeal,” Abbott said in a written statement.

The trial stemmed from a battle over stringent voter ID measures signed into law by Texas Republican Governor Rick Perry in 2011. The measure, which supporters say will prevent voter fraud, requires voters to present a photo identification such as a driver’s license, passport or military ID card.

Plaintiffs argued the law would hit elderly and poorer voters, including ethnic minorities, hardest because they are less likely to have such identification.

They would therefore be more likely to be turned away on voting day, suppressing the turnout of groups who traditionally have supported Democrats, the plaintiffs charged.

The ruling comes on the same day that the U.S. Supreme Court prevented a new voter identification law in Wisconsin from going into effect.

The nation’s top court, with three conservative members dissenting, granted a last-minute request by civil rights groups seeking to block an appeals court ruling that the law could be implemented.

Holder said in July that Texas would be the start of his push to overturn the voter ID laws.

The Obama administration sees such statutes as discriminatory and has launched a national rollout of cases to work around Shelby County v. Holder, the Alabama case in which the Supreme Court invalidated a key part of the 1965 Voting Rights Act.