A Pennsylvania judge Wednesday allowed a Republican-backed law requiring voters to show IDs to go into effect starting this Election Day, a setback for Democrats and civil rights groups who contend that such laws could deny many Americans the right to vote.

Commonwealth Court Judge Robert Simpson said those challenging the law had failed to prove that it violates the state constitution by denying voters’ rights. He also disputed the challengers’ predictions of the number of voters at stake and said there is still time for those without proper ID to acquire it.

Pennsylvania, a crucial battleground in this year’s presidential election, is also the front line of a bitter split between many Democrats and Republicans over voting rights. Supporters of recent laws passed by several Republican legislatures requiring voters to show IDs say the measures protect the integrity of the electoral process by making sure only qualified voters cast ballots.

Democrats and minority groups counter that the restrictions are likely to disenfranchise voters — often minorities, the poor and elderly, those who live in urban areas and others who may not have the type of photo ID required.

In Pennsylvania, the voter ID requirement is a “reasonable, non-discriminatory, non-severe burden when viewed in the broader context of the widespread use of photo ID in daily life,” Simpson wrote in his opinion. “The Commonwealth’s asserted interest in protecting public confidence in elections is a relevant and legitimate state interest sufficiently weighty to justify the burden.”

The detailed 70-page opinion by Simpson, a veteran of the bench and a Republican, shows the difficulty of challenging such laws before they take effect. He said the challengers, including several elderly voters who said they lack the specific documents required by Pennsylvania’s law, did an excellent job of “putting a face” on those burdened by the measure.

But he said that unless it can be clearly shown that the requirements violate the state constitution by denying voters’ rights, he is obliged to uphold the law passed by the legislature.

The case will be immediately appealed to the Pennsylvania Supreme Court. But Richard L. Hasen, an election law expert at the University of California at Irvine, said that although he disagrees with Simpson’s opinion and voter ID laws in general, he found the ruling well-reasoned and nonpartisan, and he predicted that it will be difficult to overturn.

The elected state Supreme Court is operating with only six members because one justice is suspended. A tie vote would uphold Simpson’s ruling. Although their decisions do not always follow partisan lines, the remaining justices are evenly divided between Democrats and Republicans.

Judith Browne-Dianis, co-director of the Advancement Project, one of the organizations challenging the law, called the ruling “a backwards move” for voters. “This requires hundreds and hundreds of thousands of Pennsylvania voters who lack the specific government-issued photo ID to jump through burdensome hoops to exercise their most basic legal right,” she said. “Many will not be able to vote at all.”

Gov. Tom Corbett said the state will focus on ensuring that those who want to vote have the necessary identification. And Secretary of the Commonwealth Carol Aichele, whose department oversees elections in Pennsylvania, said she welcomed the ruling.

“By giving us a reliable way to verify the identity of each voter, the voter ID law will enhance confidence in our elections,” she said.

The battle in Harrisburg is one of dozens being waged in courtrooms in Washington and across the country involving new voting laws. The Justice Department has objected to statutes in several states, including Texas, South Carolina and Florida, where changes in voting laws must be approved by federal authorities or the courts.

Cases of voter impersonation fraud — the kind that would be stopped by photo ID laws — are exceedingly rare. Pennsylvania acknowledged that such fraud had not occurred in the commonwealth, nor was it likely to occur in the coming election, even without the law.

But the state said that requiring ID is a rational way to protect the voting process and that the legislature had that right under the state constitution.

The law’s opponents said hundreds of thousands of Pennsylvanians who have voted all their lives nevertheless lack the specific kind of photo ID now required to vote, as well as the documents they need to get such an ID.

Simpson was skeptical of the challengers’ estimates. He said he believed that more than 1 percent of the state’s more than 8 million voters lack the required ID, but he did not accept the opponents’ estimate of 9 percent.

He cited pledges by the secretary of state to streamline the process for requesting IDs. With the availability of absentee voting and the acceptance of votes on a provisional basis, Simpson said, he was not convinced that any of the petitioners “will not have their votes counted in the general election.”

The law allows voters who lack the required ID to cast a provisional ballot; their vote would be counted if they could later provide the needed identification.

While the challenge was brought under the state constitution, Simpson’s opinion was heavily influenced by a 2008 decision by the U.S. Supreme Court that seemed to give states the green light to require voters to present photo IDs. In the court’s lead opinion, Justice John Paul Stevens, now retired, said that such a law in Indiana was a reasonable reaction to the threat of voter fraud, “amply justified by the valid interest in protecting the integrity and reliability of the electoral process.”

Simpson said he considered complaints that Pennsylvania’s law was motivated by partisan interests, noting what he called the “disturbing, tendentious statements” by state House Majority Leader Mike Turzai (R).

Turzai listed the law as an accomplishment at a meeting of GOP activists, saying, “Voter ID — which is going to allow Governor Romney to win the state of Pennsylvania — done.”

But Simpson said there was no proof that other lawmakers shared Turzai’s “boastful” view. Even if there were partisan motivations, the judge said, the Supreme Court’s decision in the Indiana case held that a nondiscriminatory law should not be invalidated simply because some legislators had partisan motivations.