Chris Mueller/Redux

The U.S. Supreme Court on Monday ruled that states can require voters to show photo identification before casting a ballot—a requirement critics say could deter many older, low-income and minority residents from voting. The decision could have an effect on the November elections because some 20 states have adopted this kind of legislation and others are considering it.

National surveys estimate that as many as 18 percent of Americans over age 65 do not have the requisite photo ID. AARP had joined the League of Women Voters and a host of civil rights and other advocacy groups in opposing the law.

In a case involving Indiana’s law requiring voters to show government-issued photo ID— such as a driver’s license or passport— the court ruled 6 to 3 that the law did not violate voters’ constitutional rights.

Voter identification is a highly partisan issue that breaks along party lines in virtually every state where these laws are proposed, with Republicans arguing photo ID is needed to prevent voter fraud while Democrats contend the laws are an unnecessary burden for older or poorer Americans who don’t drive or travel outside the country.

The debate is especially contentious because the very narrow voter fraud this law addresses—someone impersonating a registered voter—is rare. In Indiana, there is no record of this kind of fraud occurring.

AARP argued in its friend of the court brief, filed in favor of overturning the Indiana statute, that the photo ID law “threatens to reduce legitimate citizen participation—particularly by older voters.”  Moreover, it argued laws like this one disproportionately affect older voters because they are less likely to have the identification and other documents necessary for compliance. An Indiana AARP poll found that more than 23,000 Indiana voters over age 60 did not have the required ID to vote under the new law.

The Indiana law says those without the requisite ID may use a provisional ballot, which is counted only if the voter travels to a government office to prove his or her identity within 10 days of the election.

The majority ruling of the court, written by Justice John Paul Stevens, found that Indiana law was “amply justified” because it helped protect the integrity and reliability of the election process. In a dissenting opinion, Justice David Souter argued that Indiana’s ID law “threatens to impose nontrivial burdens on the voting rights of tens of thousands of the state’s citizens.”