Democrats fear the law will suppress turnout among their core supporters
The US Supreme Court has upheld an Indiana state law requiring people to show photo identification before voting, to guard against fraud.
Critics of the law argue that it discriminates against poor voters, ethnic minorities and the elderly, who are less likely to carry ID.
But the court ruled that the law did not constitute a burden on voters.
More than 20 other states have similar rules, and the court's ruling could prompt more states to adopt the law.
Many Democrats believe that the voter ID laws represent an attempt by Republicans to suppress turnout among groups - like African-Americans and the poor - which traditionally vote for the Democrats.
And civil rights activists say that there is little history of voter fraud in Indiana.
"This decision is a body blow to what America stands for - equal access to the polls," said Democratic Senator Charles Schumer.
But in his majority ruling, Justice John Paul Stevens argued that the law "is amply justified by the valid interest in protecting the integrity and reliability of the electoral process".
And he said that it did not "impose excessively burdensome requirements on any class of voters".
Indiana voters are able to apply for free photo ID, and voters without ID are allowed to cast provisional ballots which become valid if the voter is able to prove his or her identity within ten days of polling day.
The law will be in effect when Indiana voters go to the polls next week in the Democratic presidential primary.