US Attorney General Eric Holder, appearing Tuesday before a national NAACP convention in Houston, compared a controversial Texas voter ID law to an illegal poll tax and vowed to continue aggressively enforce voting and other civil rights laws.

“We will not allow political pretexts to disenfranchise American citizens of their most precious rights,” Mr. Holder said to the cheering crowd.

The attorney general, who was recently found in contempt of Congress by the GOP-led House, has been at the forefront of an Obama administration strategy of challenging efforts by Republican-controlled state governments in areas of voting and immigration.

The attorney general’s comments came as a panel of three federal judges in Washington heard testimony in the second day of a weeklong trial to determine whether the Texas voter ID law violates the Voting Rights Act.

The 2011 Texas law requires would-be voters to present government-issued photo ID to cast a ballot. Texas officials defend the law as an effective deterrent to voter fraud.

A number of minority groups opposed the law, and the Justice Department blocked the measure, saying it would have a discriminatory impact on minority voting rights.

“In recent months, Texas has – in many ways – been the center of our national debate about voting rights issues,” Holder said.

Last fall, the Justice Department challenged the legality of redistricting plans drawn by the state’s Republican-controlled legislature. Now, the battle is over voter IDs.

“After close review, the Department found that this law would be harmful to minority voters – and we rejected its implementation,” he said.

The attorney general said the Texas law permits concealed handgun licenses as a valid voter ID but not student IDs.

He added that a recent national study had shown that while roughly 8 percent of white voting-age citizens lack government issued photo ID, 25 percent of prospective voters who are African-American do not possess acceptable identification.

“I can’t predict the future. And I don’t know what will happen as this case moves forward,” Holder said. “But I can assure you that the Justice Department’s efforts to uphold and enforce voting rights will remain aggressive,” he said.

“The arc of American history has always moved toward expanding the electorate. It is what has made this nation exceptional,” Holder said. “We will simply not allow this era to be the beginning of the reversal of that historic progress.”

Holder also mentioned the US Supreme Court’s recent decisions upholding President Obama’s health-care reform law and striking down a portion of Arizona’s controversial immigration enforcement law.

“These monumental rulings constituted an important step forward – providing a clear and final decision on a landmark health-care law that will offer desperately needed help to millions of Americans,” Holder said, “and, in the Arizona decision, confirming the federal government’s exclusive authority to regulate on immigration issues, so that our nation speaks with one voice in this important area.”

The Arizona law, SB 1070, was passed by the state’s Republican-controlled legislature and signed by Republican Gov. Jan Brewer. State officials complained of being overwhelmed by illegal immigration and sought to counter what they viewed as lax federal enforcement of immigration laws.

The Supreme Court sided with the Justice Department and invalidated most of the law as preempted by federal statutes. But it upheld the law’s most controversial provision, the so-called show me your papers requirement.

Holder noted that he remained concerned about the high court’s decision to uphold the centerpiece of the Arizona law – the part that requires state and local law enforcement to check the immigration status of anyone they have reason to suspect might be in the US illegally.

Mr. Obama and Holder have expressed concern that the measure might lead to illegal racial profiling.

“No American should ever live under a cloud of suspicion just because of what they look like,” Holder told the NAACP convention. “Going forward, we must ensure that Arizona law enforcement officials do not enforce this law in a manner that undermines the civil rights of Americans.”

The Justice Department’s response to the Arizona ruling was swift and decisive. Within hours of the high court decision, a long-standing cooperation agreement between federal and Arizona officials on matters of immigration was unilaterally revoked.

Prior to the revocation, Arizona was one of 24 states under the federal-state cooperation agreement.