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.