In remarks to the NAACP in Houston, the attorney general said the Justice Department "will not allow political pretexts to disenfranchise American citizens of their most precious right."
Under the law passed in Texas, Holder said that "many of those without IDs would have to travel great distances to get them -- and some would struggle to pay for the documents they might need to obtain them."
Holder made the comments amid a trial in federal court in Washington over the 2011 law passed by Texas' GOP-dominated Legislature that requires voters to show photo identification when they get to the polls.
"I don't know what will happen as this case moves forward, but I can assure you that the Justice Department's efforts to uphold and enforce voting rights will remain aggressive," the attorney general said.
He said the arc of American history has always moved toward expanding the electorate and that "we will simply not allow this era to be the beginning of the reversal of that historic progress."
The attorney general spoke at the 103rd convention of the National Association for the Advancement of Colored People, which is launching a battle against new state voter ID laws. NAACP President Benjamin Todd Jealous has likened the fight against conservative-backed voter ID laws passed in several states to "Selma and Montgomery times," referring to historic Alabama civil rights confrontations of the mid-1960s.