Billionaire hedge fund manager Paul E. Singer is one of the Republican Party’s most important money men, raising millions for presidential candidate Mitt Romney and giving even more to a super PAC supporting his campaign.

But Singer is also a longtime backer of marriage rights for gay men and lesbians, putting him in stark conflict with Romney and the Republican establishment.

The contrast underscores a growing rift between the main Republican Party — which reiterated its support this week for a constitutional amendment banning gay marriage — and a small group of conservative donors who view the issue as a matter of individual civil liberties.

Singer has given $1 million this year to Freedom to Marry, a national bipartisan advocacy group focused on winning state ballot measures on gay marriage in Maine, Minnesota and Washington. The group plans to spend at least $3 million on its efforts.

Singer, founder of the $20 billion Elliott Management fund, also gave $1 million in start-up money to American Unity PAC, a new super PAC focused on supporting Republican congressional candidates who favor marriage equality.

The two groups have received major donations from at least three other Republican hedge fund managers: Cliff Asness of AQR Capital Management, Seth Klarman of Baupost Group and Dan Loeb of Third Point, according to records and officials. Singer and Asness were among the key backers of a successful push last year for same-sex marriage legislation in New York.

Marc Solomon, national campaign director for Freedom to Marry, said the donations from prominent Republicans represent a key development in the marriage debate.

“The strong support that we’re getting from members of both parties indicates that this has become a mainstream American cause,” Solomon said. “This is not the same wedge issue that it was eight years ago.”

But support for same-sex unions remains an outlier among Republicans, who included an anti-gay-marriage plank in the party’s official platform ahead of the GOP convention next week in Tampa.

The platform committee also rejected a proposal Tuesday to include a plank endorsing civil unions for gay men and lesbians. One delegate, Indiana lawyer James Bopp Jr., called same-sex unions “counterfeit marriage.”

Democrats, by contrast, have embraced gay marriage as part of their party platform after President Obama’s endorsement of marriage equality earlier this year. Gay voters make up a crucial part of the Democratic support base and have helped Obama raise tens of millions of dollars for his reelection effort.

The main focus this fall, however, will be in a handful of states where voters will decide whether same-sex marriage should be legal. Proponents have yet to win a ballot initiative, and about 30 states ban gay unions in some form.

Measures in two states, Maryland and Washington, will determine whether to repeal gay marriage laws, while Minnesota voters are considering whether to ban them as part of the state’s constitution. In Maine, meanwhile, supporters have placed a measure on the ballot that would legalize same-sex marriages and overturn an earlier ballot initiative forbidding them.

Freedom to Marry is supported by donors from both parties, including Democrats such as Facebook co-founder Chris Hughes and his husband, investor Sean Eldridge.

American Unity PAC reported to the Federal Election Commission that it has raised $1.6 million from five individuals. It describes itself on its Web site as “protecting and promoting inclusive Republicans.”

Singer, who has a gay son, has devoted an estimated $10 million toward efforts to legalize gay marriage, including the successful campaign last year in New Y ork. He declined to comment through a spokesman.

A key advocacy group leading the charge on the other side is the National Organization for Marriage, which has helped pass dozens of anti-gay-marriage measures and is devoting resources to the four ballot states this November. NOM has reported about $200,000 in independent expenditures this year, but most of the nonprofit group’s spending is not disclosed to the FEC.

NOM President Brian Brown said in a recent statement that the definition of marriage will be “a key issue” in swing states this November.

“We fully intend to make sure that people realize that the outcome of the election is a proxy for the survival of traditional marriage in our nation,” he said.

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