The battle over voting rights will expand this week when lawmakers in Missouri are expected to support a proposed constitutional amendment that would allow elections officials to require proof of citizenship from anyone registering to vote.

The measure is a far more rigorous demand than the voter ID requirement recently upheld by the Supreme Court, in which Indiana voters had to prove their identity with a government-issued card.

Sponsors of the amendment -- which would then require the approval of voters to go into effect, possibly in an August referendum -- say it would prevent illegal immigrants from affecting the political process, but critics say it could disenfranchise tens of thousands of legal residents who would find it difficult to prove their citizenship.

Voting experts say the Missouri amendment represents the next logical step for those who have supported stronger voter ID requirements and the next battleground in how elections are conducted. Similar measures are being considered in at least 19 state legislatures. The bills in Oklahoma, Kansas, South Carolina and Florida have strong support, but only Missouri's has a chance of taking effect before the presidential election.

In Arizona, the only state with a similar requirement, more than 38,000 voter registration applications have been thrown out since the state adopted its measure, called Proposition 200, in 2004, according to the election data obtained through a lawsuit filed by voting rights advocates and provided to The New York Times. More than 70 percent of those registrations came from people who stated under oath that they were born in the United States, the data showed.

Already, 25 states require some form of identification at the polls, and more may soon decide to do so now that the Supreme Court has upheld the practice. Democrats have already criticized these requirements as implicitly designed to keep lower-income voters from the polls, and are likely to fight even more fiercely now that the requirements are expanding to include immigration status.

"Three forces are converging on the issue: security, immigration and election verification," said Dr. Robert A. Pastor, co-director of the Center for Democracy and Election Management at American University in Washington. This convergence, he said, partially explains why such measures will likely become more popular and why they will make election administration, which is already a highly partisan issue, even more heated and litigious.

The Missouri secretary of state, Robin Carnahan, a Democrat who opposes the measure, estimated that it could disenfranchise up to 240,000 state residents, including citizens, who would be unable to prove their citizenship.

In most of the states that require identification, voters can use utility bills or paychecks, driver's licenses, or student or military ID cards to prove their identity. In Indiana's Democratic primary election last week, several nuns were denied ballots because they lacked the required photo IDs.

Measures requiring proof of citizenship raise the bar higher because they offer fewer options for documentation. In most cases, aspiring voters would have to produce an original birth certificate, naturalization papers or a passport.

Changes to licenses

Missouri and Arizona, along with some other states, now show whether a driver is a citizen on the face of a driver's license, and within a few years all states will be required by the federal government to restrict licenses to legal residents.

Critics say that when this level of documentation is applied to voting, it becomes more difficult for the poor, disabled, elderly and minorities to participate in the political process.

"Everyone has been focusing on voter ID laws generally, but the most pernicious measures and the ones that really promise to prevent the most eligible voters from voting is what we see in Arizona and now in Missouri," said Jon Greenbaum, a former voting rights official at the Department of Justice and currently the director of the voting rights project at the Lawyers' Committee for Civil Rights Under Law, a liberal advocacy group.

Aside from its immediacy, the action by Missouri is important because it has been a crucial swing state in recent presidential elections, with outcomes often decided by a razor-thin margin.

Supporters of the measures cite growing concerns that illegal immigrants will try to vote. They say that proof of citizenship measures are an important way to improve the accuracy of registration rolls and the overall voter confidence in the process.

State Rep. Stanley Cox, a Republican from Sedalia and the sponsor of the amendment, said the Missouri constitution already requires voters to be citizens and that his amendment simply aims to better enforce that requirement.

"The requirements we have right now are totally inadequate," he said. "You can present a utility bill, and that doesn't prove anything. I could sit here with my nice photocopier and create a thousand utility bills with different names on them."

From October 2002 to September 2005, the Department of Justice indicted 40 voters for registration fraud or illegal voting, of whom 21 were non-citizens, according to Justice Department records.