At the same time, Mayor Michael R. Bloomberg, a billionaire businessman and philanthropist who has been a major contributor to Senate Republicans in New York, plans a significant push for same-sex marriage: giving at least $100,000 of his own money, hosting a fund-raiser at an Upper East Side town house, traveling to Albany to lobby lawmakers and giving a speech on the issue.

Image Paul E. Singer, one of the most generous Republican donors in the country. Credit... Lucas Jackson/Reuters

The new donations represent roughly two-thirds of the same-sex marriage coalition’s fund-raising, making New York the rare state where a lobbying campaign in favor of legalizing gay unions is not being financed primarily by liberal donors and Democrats. The support is likely to jolt the traditional financial and political backers of gay rights causes, who now find themselves in the unfamiliar position of being outraised and outspent in New York.

The donations are financing an intensive campaign of television advertisements and grass-roots activism coordinated by New Yorkers United for Marriage, a group of same-sex marriage advocates. The campaign is aimed chiefly at persuading several members of the Senate Republican majority to join most Senate Democrats in backing same-sex marriage, which was defeated in the Senate in 2009. The State Assembly, controlled by Democrats, has repeatedly passed same-sex marriage bills.

The newly recruited donors argue that permitting same-sex marriage is consistent with conservative principles of personal liberty and small government.

“I’m a pretty straight-down-the-line small-government guy,” said Mr. Asness, who described himself as a libertarian who favored less government intrusion in both markets and personal affairs. Mr. Asness, a frequent Republican donor, has praised Tea Party activists on his blog and last year attended a conference of right-leaning donors held by Charles and David Koch, among the leading conservative philanthropists in the nation.