Five states passed laws this year scaling back programs allowing voters to cast their ballots before Election Day, the Brennan Center found. Ohio passed a law eliminating early voting on Sundays, and Florida eliminated it on the Sunday before Election Day — days when some African-American churches organized “souls to the polls” drives for members of their congregations. Maine voted to stop allowing people to register to vote on Election Day — a practice that had been credited with enrolling some 60,000 new voters in 2008. Voters in Maine and Ohio are now seeking to overturn the new laws with referendums.

The biggest impact, the Brennan Center said, will be from laws requiring people to show government-issued photo identification to vote. This year, 34 states introduced legislation to require it — a flurry of activity that Jennie Bowser, a senior fellow at the National Conference of State Legislatures, called “pretty unusual.” Before this year, only two states, Indiana and Georgia, had “strict” photo identification requirements for voters, according to the conference. This year, five more states — Wisconsin, Kansas, South Carolina, Tennessee and Texas — passed laws to join their ranks.

Under the Texas law, licenses to carry concealed handguns would be an acceptable form of identification to vote, but not student ID cards.

The Brennan Center estimates that 11 percent of potential voters do not have state-issued photo identification. By that measure, it finds that the new laws would affect 3.2 million voters in the states where the change is scheduled to take effect before the 2012 elections.

While other groups have made similar estimates in the past, Hans von Spakovsky, a senior legal fellow at the Heritage Foundation who oversaw elections at the Justice Department during the administration of President George W. Bush, argued that the number is too high.

But there is little dispute that the new laws will have an effect on a large number of voters.

South Carolina and Texas estimate that between them they have more than 800,000 registered voters who may not have acceptable forms of photo identification. While both states will offer free identification cards that would be acceptable at the polls, critics of the new laws worry that the added barrier to voting could discourage people from going to the polls. South Carolina estimates that 8 percent of its voters — 216,596 people — do not currently have the proper identification.