ALBANY – Three weeks before the Republican state convention, gubernatorial candidate John Faso is closing in on his opponent, William Weld, in the battle for party support, Republican sources say.

The party leader of Westchester County, one of the most populous Republican regions in the state, is expected today to endorse Mr. Faso, a former assemblyman who failed in his 2002 bid to be elected state comptroller.

Republicans say Mr. Faso has support from more than 30% of party delegates, more than he needs for an automatic spot on a primary ballot in September. Attention is now focused solely on Long Island, whose two counties make up about 20% of the Republican vote. Leaders from both counties are playing their cards close and are not expected to announce an endorsement until closer to the convention, which begins at the end of May.

The most likely scenario is that Mr. Weld is backed by both Suffolk and Nassau, the host of the convention. But Republicans close to Mr. Faso say the Suffolk County party may be leaning away from Mr. Weld, potentially creating a rare island split.

Six months ago, the outlook for Mr. Weld was bright. He was the chosen candidate of the leader of the Republican state committee, Stephen Minarik, who presented him to party members as an experienced and talented politician who could quickly master the politics and issues of New York and who could tap into a national fund-raising base to challenge Attorney General Eliot Spitzer.

In December, when county party chairmen met and took part in a straw poll, Mr. Weld came away with about 40% of the weighted vote, beating Mr. Faso by a 4-to-1 margin. But the counties with the most Republicans voters didn’t participate, leaving things up in the air.

Since the straw poll, Mr. Faso has won the backing of the head of the Conservative Party, Michael Long, who rejected Mr. Weld because of his relatively liberal views on social issues like gay marriage and abortion. Sources say Mr. Long put pressure on the Republican Senate majority leader, Joseph Bruno, to steer away from Mr. Weld. Mr. Long threatened to withhold his party’s endorsement of Senate Republican candidates, sources said.

Mr. Bruno this week said he would not endorse a candidate before the convention and said he would use his influence to help Mr. Faso get the minimum support needed from delegates to force a primary. Republicans say Mr. Bruno’s push for a primary has helped fuel Mr. Faso’s comeback.

The backing of Westchester comes as a new poll shows that Mr. Faso leads Mr. Weld 30% to 21% among decided Republican voters. Almost half of those polled said they hadn’t made a decision on who should be the party’s nominee. Both candidates trail Mr. Spitzer by 50 percentage points.

Mr. Faso “has the momentum. He’ll probably end up our nominee,” the chairman of the Livingston County Republican Committee and a supporter of Mr. Faso, Lowell Conrad, said.

“I know there’s a lot of talk about momentum, and I know what Joe Bruno said,” the chairman of Manhattan’s Republican Party, James Ortenzio, said. Mr. Ortenzio, who is backing Mr. Weld, said it was too difficult to predict the outcome of the convention.

Republicans backing Mr. Faso say they are more familiar with him as a politician than they are with Mr. Weld, who moved to the Upper East Side in 2000. They also say Mr. Faso’s views are more closely aligned with the party’s agenda.

Republican sources say, however, that Mr. Weld has outpaced Mr. Faso in fund-raising, taking in more than $5 million. Mr. Weld’s supporters say the former governor of Massachusetts is a superior candidate because he has more appeal among general election voters in New York City and Long Island. They point out that Mr. Faso performed poorly in those areas when he ran for comptroller against Alan Hevesi, who before winning that election had finished in fourth place in the Democratic New York City primary in 2001.

“We’re working hard,” Mr. Faso said. “We’re doing well, and I am very encouraged.”