Over the past year, the main story line in the push for marriage equality has been the ardor and success with which leading Democratic politicians have taken up the fight. The Democratic governors of New York, Maryland and Washington all promoted and signed same-sex marriage laws, for which President Obama expressed his support last month.

But the progress within Republican ranks has also been pivotal, not to mention fascinating. And a compelling character in that subplot just added a new twist to the narrative, one that suggests the rapidly changing political dynamics of this issue and its potential import to a party dogged by an image of being culturally out of touch.

That character is Paul E. Singer, 67, a billionaire hedge-fund manager who is among the most important Republican donors nationwide. In just one Manhattan fundraiser last month, he helped to collect more than $5 million for Mitt Romney’s presidential campaign.

He steadfastly supports conservative candidates. He also steadfastly supports gay rights in general and marriage equality in particular. Along with a few other leading Wall Street financiers, he contributed and helped drum up the majority of the money — more than $1 million — that fueled the campaign for same-sex marriage in New York.

He has given nearly $10 million of his own money to gay-rights initiatives, including the same-sex marriage efforts not only in New York but also in New Hampshire and New Jersey. And that figure doesn’t include his assistance in tapping a broad network of donors for individual candidates. He was pivotal in rounding up about $250,000 apiece for the Republican state senators in New York whose votes for same-sex marriage provided its margin of victory in the Legislature.

Now, Singer says, he’s providing $1 million to start a new super PAC with several Republican compatriots. Named American Unity PAC, its sole mission will be to encourage Republican candidates to support same-sex marriage, in part by helping them to feel financially shielded from any blowback from well-funded groups that oppose it.

In an interview on Tuesday, he told me that he’s confident that in congressional races, which would most likely be the super PAC’s initial focus, there are more than a few Republicans “who could be on the verge of support” or are “harboring and hiding their views.”

“And this kind of effort could be catalytic in generating some more movement,” he said.

Singer doesn’t court a high news-media profile. His willingness to meet at the midtown Manhattan offices of his hedge fund, Elliott Management, and talk about marriage equality reflects the strength of his commitment to the cause. Although he is straight, he has a gay son and son-in-law who were married in Massachusetts, which legalized same-sex marriage in 2004.

Our conversation also reflected a growing awareness among prominent Republicans that embracing marriage equality could broaden the party’s base and soften the party’s image in crucial ways. Many swing voters who find elements of Republicans’ limited-government message appealing and have doubts about Obama’s economic stewardship are nonetheless given serious pause by the party’s stances on abortion, birth control, immigration and homosexuality.

“There’s a feeling among some people that the Republican party is harsh on some things,” acknowledged Singer. Referring to opposition to same-sex marriage, he added: “Atmospherically and tonally, it’s part of the landscape of so-called harshness.”