ALBANY — Although support for gay marriage is gaining in the Legislature, lawmakers are unlikely to pass a bill this year granting legal recognition to same-sex unions.

An increasing number of Democratic lawmakers in the Senate and the Assembly have indicated they would vote to allow gay couples to wed, but opposition is strong in the Republican-controlled Senate, a roadblock that won’t necessarily disappear if Democrats take over the chamber.

Senate Democrats, who have been in lockstep with Governor Spitzer, may draw the line on gay marriage, pushing for civil unions instead. Those interviewed said a handful of members of the conference have voiced opposition to same-sex unions, a number that could be sizable enough to derail the measure.

One Democratic opponent of gay marriage, Ruben Diaz, said he estimated that 10 members of his conference would reject a gay marriage bill. Other lawmakers interviewed said the number of opponents was much smaller, perhaps as low two or three.

The Senate minority leader, Malcolm Smith, one of the more conservative members of his conference, said last fall he would be in favor of gay marriage legislation, but has said his conference has yet to agree on a position.

“It has to be negotiated,” the deputy minority leader of the Senate, Jeffrey Klein, who represents a Bronx district with a large constituency of Italian Catholics, said. “People opposed to gay marriage will be more willing to support civil unions.”

Without unanimous support among Senate Democrats, supporters of the bill would be forced to make up the difference by picking up Republican votes.

Gay marriage advocates say they are focusing their lobbying efforts on the Assembly, which has traditionally come out front on gay rights measures, and are pushing for a vote in the chamber by the end of this year’s session.

The speaker of the Assembly, Sheldon Silver, an Orthodox Jewish representative of the Lower East Side, has not publicly taken a position on the issue.

The election of Mr. Spitzer, a proponent of gay marriage, and a recent court decision placing the fate of gay marriage in the hands of the Legislature have made New York a key battleground state on the issue of same-sex unions. Gay rights activists say the state is the most likely to become the first to legalize gay marriage through a legislative process.

In July, the Court of Appeals ruled that the state constitution does not require same-sex couples to be allowed to marry but also does not prohibit lawmakers from changing the legal definition of marriage to extend it to gays.

In 2005, California’s Legislature became the first legislative body in the nation to approve gay marriage, but the measure was vetoed by Governor Schwarzenegger, who said the matter should not be decided by legislative action but by a court decision or a referendum.

Proponents expect that Mr. Spitzer will introduce a program bill legalizing same-sex unions soon after he and lawmakers agree on a budget, which is due on April 1. Spitzer officials have said the governor would propose a gay marriage bill by the end of the year, remarking in January that the issue was not on the front burner.

The Empire State Pride Agenda, a leading gay advocacy group in New York, says 54 members of the 150-person Assembly have expressed support for a gay marriage bill, up from 35 when it polled the chamber in September.

“We think it has a real shot of passing in the Assembly this year,” the executive director of the Empire State Pride Agenda, Alan Van Capelle, told The New York Sun.

One of those 54 Assembly members is Richard Brodsky, a Democrat who represents a district in Westchester. Mr. Brodsky said he senses that support for a bill is growing in his conference. “I think the prejudice is diminishing. People are thinking about it in reasonable terms,” he said.

Pride Agenda said 18 senators favor legalizing gay marriage, an increase of four from the count six months ago. There are 62 senators, 33 of which are Republican.

Pride Agenda has stepped up its lobbying efforts, more than doubling its budget and its staff size in the past three years. The group was heavily involved in the special election campaign of Democratic Senate candidate Craig Johnson, a Nassau County politician who became the first proponent of gay marriage to win a Long Island seat.

Mr. Capelle, a former political director for 1199 SEIU health care employee union, has also sought to get labor unions and federations behind the legislation. His group also expects that if Mr. Spitzer puts forward a program bill, the campaign will receive an infusion of money from national gay and lesbian groups, such as the Gill Foundation.