Correction: An earlier version of this article incorrectly said that voters in Clark County, Nev., are purged from the rolls if they are no longer living at the address on voter files. Voters are not purged from the rolls immediately after a piece of mail is returned, but they are made inactive and can be purged from the voter rolls ­after two election cycles. This version has been corrected.

The number of black and Hispanic registered voters has fallen sharply since 2008, posing a serious challenge to the Obama campaign in an election that could turn on the participation of minority voters.

In the 2008 election, robust turnout among black and Latino voters is credited with putting Obama over the top in key swing states, including Virginia and New Mexico.

Voter rolls typically shrink in non-presidential election years and registrations among whites fell at roughly the same rate, but this is the first time in nearly four decades that the number of registered Hispanics has dropped significantly.

That figure fell 5 percent across the country, to about 11 million, according to the Census Bureau. But in some politically important swing states, the decline among Hispanics, who are considered critical in the 2012 presidential contest, is much higher: just over 28 percent in New Mexico, for example, and about 10 percent in Florida.

For blacks, whose registration numbers are down 7 percent nationwide, and Hispanics, the large decrease is attributed to the ailing economy, which forced many Americans to move in search of work or because of other financial upheaval.

“The only explanation out there is the massive job loss and home mortgage foreclosures, which disproportionately affected minorities,” said Antonio Gonzalez, president of the William C. Velasquez Institute, a nonpartisan policy group that focuses on Latinos. “When you move, you lose your registration.”

Political strategists and election experts are divided on whether registrations will rise to their previous levels. But the prospect of a tight race between President Obama and Mitt Romney, the presumptive Republican nominee, has placed great importance on getting eligible Americans to register and vote.

The decline in minority registration “is obviously an area of concern,” said Democratic strategist Simon Rosenberg, president of NDN, a left-leaning think tank.

But he predicted the Obama campaign “will have enough money and enough focus to mitigate the problem. . . . They have five months to get the electorate looking the way they want.”

The GOP is also watching the shifting voter registration numbers, tracking active Republican voters in swing states and making sure they are still registered. In some places, the number of voters registered as Republicans is catching up with Democrats.

“We have really closed the gap in key battleground states,” said Republican National Committee spokeswoman Kirsten Kukowski, pointing to the relative parity Republicans have reached with Democrats in Iowa and Colorado.

The president’s reelection effort is focusing early this year on voter registration, which typically happens just before an election, between September and November. Last weekend, the campaign held a series of voter registration training sessions in a dozen states. In Florida, where the number of registered Hispanic voters has dropped by more than 140,000, the campaign trained hundreds of volunteers to sign up voters.

Those efforts, say campaign officials, have been complicated by laws approved by state legislatures since 2008, including some that place additional requirements on groups that register voters.

“It is disheartening to see voting becoming harder in states across the country,” said Katie Hogan, a spokeswoman for the Obama campaign. She said the campaign is “doing the challenging work of registering voters, even when Republican legislation is trying to make it more difficult.”

A dozen state legislatures passed rules last year requiring voters to present state-issued photo IDs when they arrive at the polls, according to the National Conference of State Legislatures, although in four states the laws were vetoed by Democratic governors. The bills continue to be a hot topic in state houses. Pennsylvania’s governor signed a voter ID bill in March, and the Virginia General Assembly also recently sent a voter ID bill to the governor.

Florida and Ohio will cut nearly in half the number of days for early voting, and Florida lawmakers reversed rules that had made it easier for former felons to vote.

Florida also passed new rules governing groups that register voters, requiring them to turn in completed voter registration forms within 48 hours or risk fines. Groups previously had 10 days to file the forms. As a result, the League of Women Voters, the Boy Scouts and several other organizations that register voters halted efforts in Florida.

Opponents of the laws say Republican legislatures have attempted to tamp down turnout among minorities, who tend to vote for Democrats.

“We’re seeing the squeeze put on voters of color. They were hit by the economy, they have to re-register to vote, and now they are hit by new registration requirements,” said Judith Browne Dianis, a civil rights lawyer and co-director of the Advancement Project, which is challenging the laws in several states.

Supporters of the laws counter that they are aimed at preventing voter fraud.

“For decades we’ve dealt with fraud and irregularities, and those concerns have to be addressed as well,” said Ana Navarro, a Florida Republican activist who supports the changes to the state’s voting laws. “If you look at almost any election in Miami-Dade, there’s always allegations of fraud.”

Together, the number of registered blacks and Hispanics across the country declined by 2 million from 2008 to late 2010, the most recent data available from the Census Bureau’s Current Population Survey.

The number of registered blacks is just over 16 million. Among whites, registrations dropped 6 percent to 104 million.

The drop is attributed not only to Americans moving from their homes, but also the assumption — by one in four voters — that their registration is automatically updated when they move, according to a report by the Pew Center on the States.

Nationally, about one in eight Americans moved between 2008 and 2010, with higher numbers among members of the armed services, young people and those living in communities affected by the economic downturn.

Nevada, a swing state with one of the highest foreclosure rates in the nation, is a prime example. In Clark County — which is 22 percent Hispanic — election officials found that more than 20 percent of voters were no longer living at the addresses on file, said David Becker, director of election initiatives for the Pew Center. Such voters can be purged from the voter rolls after two election cycles.

“Voters aren’t even thinking about it, and no one is trying to engage with them until 30 or 60 days before a presidential election,” Becker said.

Among Latinos, the decline has altered a trend of steady growth. Given that 12 million Latinos were registered to vote in 2008, some analysts had projected the number would grow to 13 million in 2010 and 14 million this election cycle. Instead, it fell in 2010 to 11 million.

“Everyone is saying the Latino vote is rocketing to the moon,” said Gonzalez of the Velasquez Institute. “It has been growing, but it stopped.”

Maria Teresa Kumar, executive director of Voto Latino — a nonpartisan group focused on registering young Hispanic voters — said the economy has made it “a lot harder to target individuals.”

But, she added, “if you just look at Latino youth, you will have 2.4 million [potential] new Latino voters. That’s three congressional districts.”

To get a jump-start, Kumar’s team is touring with Mana, a popular Mexican rock band, and collecting e-mail addresses to push voter engagement.

The NAACP will launch its voter registration campaign next week and will begin hiring field staff in a dozen states, including Florida, Georgia, Virginia and Pennsylvania. It will also mail voter registration forms to more than 1 million blacks who will turn 18 years old before the election.

“There have been a significant set of challenges that have been put before African American voters this election cycle, and us starting earlier serves to build momentum,” said Marvin Randolph, NAACP’s senior vice president for campaigns.

Staff writer Laura Vozzella contributed to this report.