Divisions among Supreme Court justices in their decision Monday to uphold Indiana’s law requiring stricter rules for voter identification likely will lead to more legal uncertainty among the states, an expert says.

The 6-3 decision upholding the Republican-backed legislation could spur other states to enact more restrictive identification requirements before November.

But because justices in the majority were split about the correct standard for reviewing voting regulations, the legal fate of other restrictive state voter ID laws is uncertain.

Proponents of the Indiana law at issue in the case, Crawford v. Marion County Election Board, said the state’s interest in preventing voter impersonation fraud by imposing tight requirements on voters to show photo identification outweighs whatever burden might be placed on a small number of people who do not have the requisite identification. The high court agreed.

“There is no question about the legitimacy or importance of the state’s interest in counting only the votes of eligible voters,” Justice John Paul Stevens wrote for the majority.

The majority, however, splintered in upholding the earlier decision by the U.S. Court of Appeals for the 7th Circuit. Stevens’ opinion was joined by Chief Justice John G. Roberts Jr. and Justice Anthony M. Kennedy. Justice Antonin Scalia wrote a concurring opinion, which Justices Clarence Thomas and Samuel A. Alito Jr. joined. They agreed with the outcome but suggested that they might allow for more restrictions based on the “minimal” burden placed on potential voters in the Indiana case.

“Without a majority opinion, it’s anyone’s guess how the next case should be decided,” said Dan Tokaji, an election law professor at Ohio State University.

In its decision, the court signaled a willingness to uphold state election regulations aimed at preventing fraud, whether or not it has actually occurred, and regardless of potential partisan motivations.

Each state sets its own rules for how elections for both statewide and federal offices are held. Those regulations have become an increasingly partisan issue since 2000, when the disputed ­Florida results kept the outcome of the presidential election in doubt.

Republicans have pushed to enact statutes aimed at what they contend is fraudulent voting among people not qualified to cast ballots. Democrats have focused on what they see as a need to curb intimidation of voters and combat efforts aimed at suppressing minority voting.

According to Rick Hasen, a professor at Loyola Law School in Los Angeles who filed an amicus brief against the Indiana law, every state voter identification law since 2000 has been enacted along party lines.

Democratic National Committee Chairman Howard Dean said the Indiana law demonstrates “the lengths Republicans will go to in their attempts to limit voting rights in order to win elections.”

Bradley A. Smith, a former Republican chairman of the Federal Election Commission, said: “The court recognized that states have a legitimate interest in fighting voter fraud and that not every minor ‘burden’ on voting constitutes ‘disenfranchisement.’ ”

In 2002, Congress passed the Help America Vote Act (HAVA, PL 107-252), which set a minimum floor for the kind of identification states must require for residents to register and vote. Voters who register by mail are required to present written identification at the polling place, including either photo identification or other forms of documentation.

That law left states free to enact stricter identification requirements, as Indiana did. Twenty-five states currently have stricter voter identification laws than required by HAVA, including seven that require photo identification, according to the National Conference of State Legislatures.

Indiana and Georgia, the states with the strictest requirements, allow voters without photo identifications to cast a provisional ballot but must return within a few days of the election and show a photo identification in order for that ballot to be counted.

GOP lawmakers in other states such as Kansas and Texas may try to enact similar requirements for the November election, Hasen said. Stricter requirements would likely result in more provisional ballots cast by voters who show up at the polls without proper identification. In any disputed contest, candidates are likely to go to court to challenge the validity of those provisional ballots.

Opponents argued that the Indiana law unfairly burdens poor and elderly voters, who tend to vote Democratic but are least likely to have photo IDs and may find it difficult to return later to show proper identification. But the challenge to the Indiana law was a “facial challenge” to its constitutionality that began right after it was enacted. The law’s opponents did not offer sufficient evidence, the court said, of unreasonable burdens placed on specific voters.

Opponents of the Indiana law or of similar laws in other states can still mount “as-applied” challenges to the laws after elections.

Justice David H. Souter, dissenting for himself and Justice Ruth Bader Ginsburg, said the burden imposed on voters outweighed the state’s interests in preventing fraud. “The law imposes an unreasonable and irrelevant burden on voters who are poor and old,” Souter wrote. Justice Stephen G. Breyer dissented separately.