In 2005, Republican lawmakers in Indiana passed the mandate that anyone who votes in person show a current, government-issued ID with a picture as a voter-fraud deterrent.
Although 24 states have enacted similar laws in recent years, Democrats and civil rights groups have challenged the Hoosier State law as unconstitutional, calling it an effort to discourage elderly, poor and minority voters -- groups who are most likely to lack proper ID and tend to vote Democratic.
Since the dispute over the 2000 presidential election, Republicans in many states have pushed for voter ID laws to stave off what they see as a significant problem: voter fraud through voter impersonation at the polls.
But studies have shown the problem does not exist, National Public Radio's Nina Totenberg reported.
Tova Wang, a democracy fellow at The Century Foundation, co-authored research and filed a federally mandated report on the question.
"We found that although there is fraud in the system, it doesn't take place at the polling place," Wang told NPR.
Voter fraud occurs in more systematic ways: ballot box stuffing, absentee balloting and manipulation of voting machines and registration lists, experts said.
Indiana Solicitor General Thomas Fisher told the court that the vast majority of Indiana voters easily comply with the law. "You're talking about an infinitesimal portion of the electorate that could be burdened," Fisher said under sharp questioning from Justice David Souter.
But with little evidence of fraud or of voters who have been kept from voting, Justice Samuel Alito said, "The problem I have is, where do you draw the line? There is nothing to quantify the extent of the problem or the extent of the burden."
Chief Justice John Roberts, who grew up in Indiana, and Justice Antonin Scalia indicated strong support for the state law, the Associated Press reported. Justice Clarence Thomas said nothing, but most often votes with his conservative colleagues. The case seemed to break along familiar ideological lines, with the court's four liberal justices more critical of the state.
Souter and Justice John Paul Stevens both pressed Fisher on the state's voter-registration problems, which Fisher acknowledged are among the nation's worst in terms of keeping the names of the dead and those who have moved on the rolls. Indianans are not required to show photo ID to register.
The state's "policy is to have it tougher to vote than to register?" Stevens asked. "That doesn't make sense to me."
The Census Bureau and the Federal Highway Administration estimate that 11 percent of voting-age citizens, some 21 million Americans, lack any form of current government-issued photo ID, NPR reported. Those without driver's licenses include many city dwellers who use public transportation, people too poor to own a car and senior citizens who no longer drive.
The court's decision likely will be issued before it adjourns at the end of June, in time for the 2008 general elections, the Washington Post reported.