HEMPSTEAD – Republican leaders in New York are pinning their hopes to beat Eliot Spitzer on John Faso, a deeply conservative and loyal party activist who is best known for his mastery of budget arcana.

Yesterday at the Republican state convention, Mr. Faso, 53, scored a remarkable come-from-behind victory against William Weld, 60, a Harvard-educated former governor of Massachusetts who was propped up by the party’s top officials but failed to impress grassroots members.

The results of the convention at Hofstra University were a rebuttal to the leadership of the party’s chairman, Stephen Minarik, who made no secret that he was one of Mr. Weld’s most fervent advocates.

By choosing Mr. Faso, a fiscal and social conservative, over Mr. Weld, delegates also signaled a rightward shift of a party that for more than a decade has followed the ideological twists and turns of Governor Pataki’s administration.

“New Yorkers are crying out for change,” Mr. Faso said after accepting the party’s designation. “They know something is wrong with the state. And I plan to let them know over and over and over again that there will be a significant choice in this election.”

Mr. Minarik made the case that Mr. Weld has a broader appeal that is critical in a state where Republicans are heavily outnumbered by Democrats. But delegates said they were more comfortable with Mr. Faso and thought his politics were more aligned with those of the party.

Mr. Faso, who served as minority leader in the state Assembly before narrowly losing a race for state comptroller in 2002, won 61.2% of the delegate vote. Mr. Weld claimed 38.8%, more than the 25% that he needed to qualify for a place on the September primary ballot.

The winner of the primary will receive the party’s nomination and face off against the Democratic nominee, which is likely to be state attorney general Eliot Spitzer, who leads all his rivals in polls and fundraising by hefty margins.

After the roll call was completed and the lopsided result was announced, Mr. Weld vowed to remain in the race and calmly told a swarm of reporters and cameras that he would topple Mr. Faso in the election in three months. Mr. Faso, who has also won the endorsement of the Conservative Party, said the support from delegates showed that he was the strongest candidate. He said he preferred there wasn’t a primary but stopped short of urging Mr. Weld to step aside.

New York political history is littered with examples of Republican and Democratic convention winners who are later rejected by primary voters. The best known example is Governor Cuomo’s primary victory over Mayor Koch in 1982.

Addressing the more than 400 delegates seated on the floor of a largely empty campus arena, Mr. Faso’s speech was laser-focused on fiscal issues. He promised to “do something” about income, property, and business taxes and to put spending “on a diet.”

Mr. Faso served as Mr. Pataki’s budget adviser during the governor’s first term and delegates said they were taken by his grasp of even the minutiae of state fiscal policy.

He also unleashed criticism of Mr. Spitzer, accusing him of planning to raise taxes and spending. “Candidate Spitzer has refused to rule out higher taxes. Amazing, remarkable, frightening,” he said. “When a candidate won’t rule out raising taxes before the election, you can bet your dollar that he’ll raise taxes in a New York minute after the election.”

He continued: “I’ll tell you the problem with Eliot Spitzer…He actually does believe in big government.”

A spokeswoman for Mr. Spitzer’s campaign, Christine Anderson, said the attorney general was “against” raising taxes, saying in an e-mailed statement that the “shrill, partisan accusations that we heard today have nothing to do what Eliot Spitzer has said, how he has governed, or where he plans on taking the state.”

Mr. Weld, who also addressed delegates, wryly described Mr. Spitzer as an “able” spokesman for the “politics of stagnation.” He also foreshadowed his attack strategy against Mr. Faso, targeting his Republican rival with innuendo that belittled Mr. Faso’s career accomplishments.

“I have a record of achieving. Not of trying, as a legislative backbencher, but of succeeding, as a chief executive in a political environment inhospitable to reform. Don’t confuse effort with results,” he said. Mr. Faso served as an assemblyman from 1986 to 2002, after which he worked as a lobbyist.

Just months ago, Mr. Weld’s campaign brimmed with confidence that it had this convention locked up, along with the expectation of endorsements from Mayor Giuliani, Governor Pataki, and the party’s other leading lights. Mr. Giuliani had encouraged him to run and Mr. Pataki, a friend of Mr. Weld’s, said he would make an “outstanding candidate.” And in December, Republican county leaders met in Albany and voted overwhelmingly in favor of Mr. Weld in an informal straw poll.

Delegates said Mr. Faso pursued their support more aggressively and many said they thought Mr. Weld was more concerned with raising money and planning strategy than courting county leaders.

If Mr. Weld, a man who gives off an air of effortless ease, was nervous coming into the convention, he didn’t let it show. In fact, as he noted in his convention speech, he spent Sunday climbing Mount Marcy and “rowing and portaging out of the lakes and forest.”

For Mr. Weld, it’s 1990 all over again. That year, delegates rejected him at the Massachusetts Republican convention, favoring a candidate with more right-wing views on abortion. Conservative party members didn’t trust him and the Republican state party chairman urged him to drop out of the race and run for attorney general. He went on to win the primary by a more than 15-point margin.

Mr. Weld is expected to roll out his first television ads within the next two weeks. A televised debate between the two candidates is scheduled for June 22.

The deadline for registering to vote in the September 12 primary is 25 days prior to the election.