President Obama’s reelection campaign launched a national drive Friday to counter new restrictive voter-access laws, which advisers said threaten his electoral chances in November.

Organizers will fan out in key swing states this weekend to teach volunteers and voters how to navigate a series of laws passed by Republican-controlled state legislatures imposing stricter identification requirements, limiting early voting and making it harder to organize voter-registration drives.

It is the beginning of a months-long effort, campaign officials said, to combat what they described as a Republican effort to stifle voting among young people and minorities, two groups that traditionally tend to vote Democratic.

Republicans say the new laws are needed to protect against voter fraud and help make elections fairer.

The Obama campaign’s “weekend of action” is part of a field effort that in 2008 helped identify, register and turn out millions of new voters. Those new voters gave Obama wins in unlikely places, including North Carolina and Virginia, where young and minority voters helped make the difference. Turning out those voters again this year is key to the president’s reelection strategy, but it is also more challenging this year in part because of the new voting laws.

“Over the past century, we expanded this fundamental right, making sure no one’s race, gender or economic status is ever used to deny this fundamental right,” said campaign strategist Michael Blake in a call with reporters Friday afternoon.

“Unfortunately,” he added, “Republican-controlled legislatures in many states have been taking us backward, not forward.”

In 2011, more than 30 states debated changes to their voting laws. A dozen passed more restrictive rules requiring voters to present state-issued photo IDs, according to the National Conference of State Legislatures, although Democratic governors in four states vetoed them. Florida and Ohio cut the number of days for early voting by nearly half, and Florida lawmakers reversed rules that had made it easier for former felons to vote.

On Friday, Virginia Gov. Robert F. McDonnell (R) signed the latest — a new voter-ID bill. Virginia already required voters to present valid identification at the polls, but those who didn’t have it were allowed to sign an affidavit pledging that they were who they said they were. Now, voters can present a wider range of valid identifications, but the affidavit option is gone, and voters without legal identification must vote provisionally and prove later that their vote should count.

Republicans in several states have said the new laws are necessary to prevent voter fraud and make elections fairer. In a statement Friday, McDonnell praised his state’s new law for “helping to further prevent voter fraud and ensuring Virginians that they can have faith that votes have not been fraudulently cast.”

But as governor of a critical swing state with a large swath of moderate voters, McDonnell is cognizant of the possibility that the law may be seen as a vehicle for voter suppression. Earlier this year, he unsuccessfully lobbied fellow Republicans in the legislature to allow for a signature comparison for voters with no ID. And on Friday, he issued an executive order calling on the state elections board to issue a registration ID to all voters so that “on election day this year, every Virginia voter will have at least one valid ID.”

Voting rights groups and Democrats, meanwhile, have decried these measures as deliberate attempts to suppress voters and swing elections. They say there have been few cases of voter fraud, given the millions of ballots cast and compared to the high number of poor and minority voters who will be affected. Some of these groups have brought lawsuits against some of the new legislation.

Twenty-five percent of African American voters do not have a valid government-issued photo ID, compared with 8 percent of whites, according to a study by the Brennan Center for Justice at New York University’s law school. The report also found that 15 percent of voters earning less than $35,000 per year do not have such an ID.

According to Project Vote, a voting-rights advocacy group, about 15,000 people voted without identification in Virginia in 2008.

A centerpiece of Obama’s new effort is a Web site, www.gottavote.org, that helps voters register and understand the voting requirements in their state. The site — and its Spanish-language version, votemostodos.org — also urges viewers to sign up to volunteer and solicits lawyers to help with voter-protection efforts.

The campaign will hold hundreds of events in key battlegrounds Saturday and Sunday. In the Washington region, mostly in the Virginia suburbs, there were 38 voter-registration events scheduled between Friday and Sunday.

A particular emphasis will be placed on training volunteers who will be deployed through the summer and fall to continue the education and registration effort directly with voters.

Staff writer Krissah Thompson contributed to this report.