Miami - Less than 18 months before the next presidential election, Republican-controlled statehouses around the country are rewriting voting laws to require photo identification at the polls, reduce the number of days of early voting or tighten registration rules.

Republican legislators say the new rules, which have advanced in 13 states in the past two months, offer a practical way to weed out fraudulent votes and preserve the integrity of the ballot box. Democrats say the changes have little to do with fraud prevention and more to do with placing obstacles in the way of possible Democratic voters, including young people and minorities.

Gov. Scott Walker of Wisconsin and Gov. Rick Perry of Texas signed laws last week that would require each voter to show an official, valid photo ID to cast a ballot, joining Kansas and South Carolina.

In Florida, which already had a photo law, Gov. Rick Scott signed a bill this month to tighten restrictions on third-party voter registration organizations — prompting the League of Women Voters to say it would cease registering voters in the state — and to shorten the number of early voting days. Twelve states now require photo identification to vote.

The battleground states of Ohio and Pennsylvania are among those moving ahead on voter ID bills, part of a trend that seems likely to intensify the kind of pitched partisan jousting over voting that has cropped up in recent presidential races.

When voters in predominantly black neighborhoods in Florida saw their votes challenged in the contested Bush-Gore election of 2000, Democrats made charges of disenfranchisement. In 2008 Acorn, a group organizing minority and low-income communities, became a particular target, with Republicans asserting that Acorn was trying to steal the election with large voter-registration drives, some of which were found to be seriously flawed.

Democrats, who point to scant evidence of voter-impersonation fraud, say the unified Republican push for photo identification cards carries echoes of the Jim Crow laws — with their poll taxes and literacy tests — that inhibited black voters in the South from Reconstruction through the 1960s. Election experts say minorities, poor people and students — who tend to skew Democratic — are among those least likely to have valid driver’s licenses, the most prevalent form of identification. Older people, another group less likely to have licenses, are swing voters.

Republicans argue that the requirements are commonplace.

“If you have to show a picture ID to buy Sudafed, if you have to show a picture ID to get on an airplane, you should show a picture ID when you vote,” Gov. Nikki Haley said this month when she signed the bill into law in South Carolina, using a common refrain among Republicans.

Changes to voter law tend to flow and ebb with election cycles as both Democrats and Republicans scramble to gain the upper hand when they hold power. The 2010 midterm election was a boon to Republicans, who now control 59 chambers of state legislatures and 29 governorships. In some states, like Florida and Texas, Republicans hold overwhelming majorities. This has allowed the bills to move forward.

Republicans have tried for years to get photo identification requirements and other changes through legislatures, said Daniel Tokaji, a law professor at Ohio State University and an expert in election law. Similar bills were introduced over the past decade, but were largely derailed in the aftermath of a political battle over the Bush administration’s firing of several United States attorneys whom Republicans had criticized for failing to aggressively investigate voter fraud.

“That’s what really killed the momentum of more states’ enacting voter ID laws,” Mr. Tokaji said. “Now with the last elections, with the strong Republican majorities in a lot of states, we’re seeing a rejuvenation > Republicans say that large jumps in the immigrant population have also prompted them to act to safeguard elections.

“Over the last 20 years, we have seen Florida grow quite rapidly, and we have such a mix of populations,” said State Representative Dennis K. Baxley, the Florida Republican who wrote the law to tighten third-party registration here. “When we fail to protect every ballot, we disenfranchise people who participate legitimately.”

Taken together, the state-by-state changes are likely to have an impact on close elections, Mr. Tokaji said.

“Remarkably, most of these significant changes are going under the radar,” he added. “A lot of voters are going to be surprised and dismayed when they go to their polling place and find that the rules have changed.”

Most of the measures would require people to show a form of official, valid identification to vote. While driver’s licenses are the most common form, voters can also request free photo IDs from the Department of Motor Vehicles or use a passport or military identification, among other things.

But Democrats say thousands of people in each state do not have these. The extra step, they add, will discourage some voters who will have to pay to retrieve documents, like birth certificates, for proof to obtain a free card. If voters do not have the proper identification on Election Day, they can cast provisional ballots in most states but must return several days later to a local board of elections office with an ID.

A few state bills and laws also shave the number of early voting days, a move that Democrats say would impact Democratic voters once again. In the 2008 presidential election, a majority of those who cast early votes did so for President Obama. In Florida, the number of days is reduced but the number of hours remains the same.

Democrats point to state figures showing that there are few proven cases of voter impersonation and question why budget-conscious Republicans would want to spend taxpayer dollars on a problem that is isolated.

“There is not one documented case that has been presented to us, and we had numerous hearings,” said State Senator Brad Hutto of South Carolina, a Democrat. “Republicans have to have some reason to do this because it doesn’t sound good to say, ‘We don’t want Latinos or African-Americans voting.’ ”

But Republicans counter that detecting and proving voter impersonation is tricky under current law precisely because few states require photo identification. Plus, they add, there is no evidence that the requirement reduces minority participation. In Georgia, where photo IDs became a requirement in 2007, minorities voted in record numbers in 2008 and 2010.

Turnout among Hispanic voters jumped 140 percent in the state in 2008 and 42 percent among blacks compared with 2004, a change attributed in part to President Obama’s candidacy. Two years later, in the midterm election, turnout also rose among Hispanics and African-Americans, according to data from the Georgia secretary of state.

But with the presidential election campaign season already under way, Democrats say they are taking no chances. The Democratic Governors Association started a Voter Protection Project this month to educate voters and encourage them to speak out against the measures. It also began running online advertisements.