President Barack Obama's Democratic allies this week ramped up attacks on what they charged are unfairly restrictive voting laws that could swing key November 2012 elections to Republicans.

House of Representatives Democrats, led by their number-two leader Steny Hoyer, announced Thursday they were writing to the top election officials in all 50 states urging them not to make it harder for eligible voters to cast ballots.

"Voting hours, voting sites, identification requirements, voter registration regulation and access to mail ballots should not be used as weapons to achieve a preferred electoral outcome," they wrote.

Democrats point to an October study that found upwards of five million eligible voters could be affected by rules put in place in a dozen states after Republicans won control of their legislatures in the 2010 mid-term elections.

The report, crafted by New York University Law School's Brennan Center for Justice, cites new requirements for voters to show government-issued photo identification, something it estimates as many as one in ten voters do not have.

It also notes shortened periods of early voting before election day, tougher requirements to register to vote or to sign up new voters, and laws limiting voting rights for convicted criminals who have served their sentence.

The states affected, including the critical battlegrounds of Florida and Ohio, account for 171 of the 270 electoral college votes needed to win the presidency, the center said in its report.

Supporters say the new rules will prevent illegal immigrants from voting, or thwart attempts to cast ballots in multiple states -- though independent and government reports do not back up claims of epidemic voter fraud.

"I have no desire to suppress people from voting," Republican Senator Lindsey Graham of South Carolina -- a state that enacted a photo-ID law this year -- said at a September 9 congressional hearing on the laws.

But "when it comes to voting, I don't think it's too much of a stretch to say you have to prove that you are who you say you are, and we'll find accommodating ways to get there," he added.

Foes of the new rules say they disproportionately affect the elderly, young voters, and minorities like African-Americans -- 25 percent of whom, the center estimated in a 2006 study, lack government-issued photo identification, and nine out of ten of whom voted Democratic in the past two presidential elections.

The issue stokes special passions in the US South, where "Jim Crow" laws, poll taxes, literacy tests and other measures served to suppress the black vote for nearly a century until abolished by the 1965 Voting Rights Act.

Democrats unhappy about the new rules point to the case of Dorothy Cooper, 96, who has voted for seven decades without any trouble but struggled this year to get the necessary photo identification in the state of Tennessee.

Cooper was reportedly twice turned away -- first, because she brought her 1915 birth certificate but lacked a marriage certificate to verify her name change, and a second time when she brought her birth certificate, a telephone bill, her lease, and marriage certificate but not her Social Security card.

She was ultimately successful after her situation made national headlines.

Democrats also cite the case of a civics teacher in Florida, the vote-rich state at the center of the disputed 2000 election, whose drive to pre-register students has fallen afoul of new rules there requiring third-party registrants to sign up with the state and to submit applications within just two days.

Democratic Senator Bill Nelson, who represents Florida and faces reelection in 2012, asked the US Justice Department on Thursday to investigate whether the new rules there and 13 other states aim "to suppress the national vote."

"The Department needs to determine whether or not there was broad-based motivation to suppress the vote -- and, if so, whether any laws were violated," he said in a letter to Attorney General Eric Holder.

The Wall Street Journal reported this week that Obama's reelection campaign had quietly launched its own counterattack, pointing to what the campaign calls a volunteer-driven drive to freeze the new rules in Ohio.