Bill Smith, deputy executive director of the Gill Action Fund, a political organization that supports gay rights, called the contributions from Republican donors “a sea change.”
Some of the donors were recruited by Ken Mehlman, a contributor to the coalition and a former chairman of the Republican National Committee, who has since announced that he is gay. Some of the new contributors have been active in national politics and presidential fund-raising, but are venturing into state politics for the first time. Their involvement offers Republicans in New York the prospect of help in next year’s legislative elections and a potential source of longer-term support for a party that has struggled to field well-financed candidates for statewide office in recent years.
Mr. Bloomberg, a Republican-turned-independent, is the most politically centrist of the donors. (He is not coordinating his efforts with the other contributors.)
The mayor has long supported same-sex marriage and spoke passionately about the subject in 2009, when the Legislature last tackled the topic. But his donations to state Republicans have occasionally stirred skepticism among gay rights advocates. And until now, the mayor has given little of his personal fortune to the cause — he made a $5,000 donation to the Human Rights Campaign, a national gay rights group, in 2010.
This year, his involvement has deepened considerably. Aides to Mr. Bloomberg said he viewed the marriage issue in a larger context: Freedom, he argues, is New York’s “competitive advantage” and its brand, and he has become committed to vigorously defending it, as he did amid criticism of a proposed Islamic center near ground zero.
 “At the core this very rational mayor is somebody who believes that government has no business in getting involved in, taking sides in or making value judgments about who you love,” said John Feinblatt, the mayor’s chief policy adviser.
On Tuesday, Mr. Bloomberg is scheduled to travel to Albany to lobby Republican lawmakers, and on May 25, he plans to hold a high-dollar fund-raiser, featuring the singer Rufus Wainwright, to raise money for the same-sex marriage cause at the Upper East Side headquarters of his foundation. The next day, Mr. Bloomberg plans to deliver a speech on the subject at the Cooper Union.
“This is the moment,” Mr. Feinblatt said of the mayor’s involvement. “If you want your beliefs to count and your voice to count, this is the time.”