WASHINGTON — A state judge refused to block a new Pennsylvania law that requires voters to display a current government-issued photo identification at the polls, upholding a Republican-backed measure that Democrats say may prevent tens of thousands of low-income and elderly voters from casting ballots in November.

The decision is a setback for voting-rights advocates who sued on behalf of a dozen, mostly elderly, voters who do not drive and do not have an ID card that will allow them to vote.

Judge Robert Simpson, who held a trial on the issue, said he was not convinced the photo-ID rule will prove an insurmountable barrier and added he was reluctant to strike down a law passed by the Legislature.

He also said the state was taking steps this summer to help voters obtain the required identification.

"I am not convinced any qualified elector need be disfranchised by Act 18," he wrote, citing the law's legislative title.

Simpson didn't rule on the full merits of the case, only whether to grant a preliminary injunction stopping it from taking effect next month.

Voters who cannot obtain a photo ID may be able to cast an absentee or provisional ballot, he said. The judge, who was elected as a Republican, said he was convinced the state will implement the law "in a nonpartisan and evenhanded manner."

His opinion relied heavily on a U.S. Supreme Court ruling that upheld a similar photo-ID law in Indiana four years ago.

The lead plaintiff, Viviette Applewhite, 93, says she has voted in every presidential race since the days of Franklin Roosevelt. She has several ID cards, including some with her photo, but she does not have a driver's license or a valid passport. Her polling place is next to her apartment building.

State officials say she could obtain a valid ID card if she brought her birth certificate, a Social Security card and a proof of residence to an office of the state Department of Transportation.

Both parties acknowledge voter turnout could play a key role in what many predict will be a tight race between President Obama and Mitt Romney, the expected Republican nominee, especially in battleground states such as Pennsylvania. Other court cases under way include federal inquiries into voter-ID laws in Texas and South Carolina and a state challenge in Wisconsin. In Ohio, a dispute over rules for early voting ended Wednesday when the secretary of state set uniform hours statewide.

Before the trial, Pennsylvania's lawyers conceded they were "not aware of any incidents of in-person voter fraud in Pennsylvania" and agreed it was not "likely to occur in November of 2012."

The two sides differed on the potential impact of the law. In March, Republican leaders estimated 1 percent of Pennsylvania's voters — or about 90,000 people — lacked the required ID cards. But in July, the state reported about 9 percent of its registered voters — more than 758,000 people — did not have a valid ID issued by PennDOT, the transportation agency.

The challengers said they would appeal to the state Supreme Court, which is operating with only six members, because one justice is suspended. A tie vote would uphold Simpson's ruling.

Material from The Washington Post and The New York Times is included in this report.