In 1964, the American people enacted the 24th Amendment to prevent the exclusion of the poor from the ballot box. In his recent speech at the NAACP convention, U.S. Attorney General Eric H. Holder Jr. wasn't indulging in election-year rhetoric when he condemned Texas' 2011 voter-identification law as a poll tax that could exclude the poor. He was speaking the hard legal truth.

The Justice Department would be right to challenge this new law as an unconstitutional poll tax. The department has temporarily blocked the Texas law under special provisions of the Voting Rights Act that apply to jurisdictions, mostly in the South, with a history of discriminating against minority groups. But the attorney general should go further and raise a 24th Amendment challenge to Texas and other states that are joining the effort to bar the poor from the polls. (The Justice Department has opened an investigation into Pennsylvania's voter-ID law.) This exclusionary campaign should not be allowed to destroy a great constitutional achievement of the civil rights revolution.

The 24th Amendment forbids the imposition of "any poll tax or other tax" in federal elections. Texas' law flatly violates this provision in dealing with would-be voters who don't have a state-issued photo ID. To obtain an acceptable substitute, they must travel to a driver licensing office and submit appropriate documents, along with their fingerprints, to establish their qualifications. If they don't have the required papers, they must pay $22 for a copy of their birth certificate.

If they can't come up with the money for the qualifying documents, they can't vote. The 24th Amendment denies states the power to create such a financial barrier to the ballot box.

Texas' violation is particularly blatant. In drafting its law, the Legislature rejected a provision that would have provided free copies of the necessary documents. Rather than paying for this service out of the general revenue fund, it chose to disqualify voters who couldn't pay the fee. This is precisely what is forbidden by the Constitution.

Unnecessarily arduous

The 24th Amendment doesn't only invalidate the $22 tax. States also can't impose unnecessarily arduous certification procedures.

The Supreme Court took up this issue shortly after the amendment was ratified in 1964. Virginia had told its citizens that they could avoid its $1.50 poll tax only if they filed a formal certificate establishing residency. Some residents refused to comply, and in 1965, a near-unanimous Supreme Court agreed with them.

Chief Justice Earl Warren wrote in the ruling that the state's administration of its residency certificate requirement was a "real obstacle to voting in federal elections" that "abridged" the franchise. He emphasized that constitutional end runs were not permitted. "For federal elections," he explained, "the poll tax is abolished absolutely as a prerequisite to voting, and no equivalent or milder substitute may be imposed."

This broad functional view of taxation is firmly rooted in our constitutional tradition. In his recent opinion in the health-care case, for example, Chief Justice John Roberts adopted the same approach in finding that the "penalty" imposed by the Affordable Care Act was the functional equivalent of a tax.

But in Warren's ruling, the same broad approach to taxation led to a very different conclusion. Unlike Roberts, Warren was not marking out the boundaries of congressional power. He was restricting the power of the states to impose unnecessary administrative barriers that were the functional equivalents of poll taxes.

Big implications

Applying Warren's approach to the present day has large practical implications. The estimated number of registered voters in Texas without valid IDs ranges from 167,000 (according to the state) to more than a million (according to the federal government). The Justice Department also emphasizes that minority groups are disproportionately affected. What's more, 10 other states — including Pennsylvania — have passed similar laws in the last two years alone. All these statutes raise fundamental problems under the 24th Amendment.

Curiously, these problems have been overlooked in the escalating wave of challenges to this recent round of exclusionary legislation. Civil rights lawyers have focused instead on more familiar texts, such as the Voting Rights Act and the 14th Amendment. Though these provisions are important, they were created in response to a host of other issues. The poll-tax amendment, in contrast, was focused on the very problem that now threatens again to undermine our democracy: imposing costs on the poor that prevent them from voting.

The attorney general was right to recall the amendment from legal obscurity, and to insist that we remember the determined efforts of the civil rights generation to end this disgraceful practice forever.