Nineteen states have laws that require voters to show photo identification at the polls, though legal fights are pending over the issue in four states, according to the National Conference of State Legislatures. Civil rights groups argue such laws unfairly target poor and elderly residents who are less likely to have IDs, while supporters say they protect against voter fraud.
Here's a look at where the legal challenges stand:
A federal appeals court reinstated Wisconsin's voter ID law on Friday after the three Republican-appointed judges questioned whether the law was truly discriminatory. Afterward the state elections board said it would move toward implementing the law in time for November's general election, which includes a tight governor's race between Republican incumbent Scott Walker and Democratic challenger Mary Burke.
Wisconsin's measure requiring voters to show photo identification was signed into law in 2011 by Walker, and it was in place during the February 2012 primary election. A county judge ruled it unconstitutional a month later, and it hadn't been in effect since.
The state Supreme Court ruled in July that the law was constitutional, but the law remained on hold because a federal judge had struck it down earlier this year.
The U.S. Justice Department is asking a federal judge to throw out Texas' voter ID law in a trial that began this month, but a ruling isn't expected until after the November elections. That means voters must show a photo ID at polling sites.
Texas became the first test by U.S. Attorney General Eric Holder to wring protections from a weaker Voting Rights Act after the U.S. Supreme Court gutted the landmark 1965 law last year. The state had been barred from enforcing voter ID requirements prior to that ruling.
Opponents say 787,000 registered voters in Texas lack any of seven acceptable forms of ID under the law. For example, concealed handgun licenses are accepted at polling sites, but not student IDs issued by state universities.
Arkansas' voter ID law was approved last year after the Republican-led Legislature overrode a veto from Democratic Gov. Mike Beebe. Although a judge ruled the law unconstitutional in May, he put his ruling on hold pending court appeals, meaning the law remains in effect.
A lawsuit challenging the law is set to go before the Arkansas Supreme Court on Oct. 2, less than three weeks before early voting begins for the November election. The American Civil Liberties Union of Arkansas is suing the state on behalf of four voters who, according to the ACLU, are being disenfranchised by the photo ID requirement. The state is asking the court to uphold the law.
A law requiring North Carolina voters to show photo identification is set to take effect in 2016, but that requirement is among several provisions in the state's 2013 elections-overhaul law that are being challenged by federal lawsuits. For now, poll workers ask voters whether they have a photo ID and give them information about how to obtain one if they don't.
Lawsuits challenging that and other law provisions are set for trial next summer, though the 4th U.S. Circuit Court of Appeals is scheduled to hear arguments this month about whether other voting restrictions can be used for the November election. If plaintiffs are successful, that could prevent poll workers from even asking voters if they have a photo ID.