Mr. Hoffman, though running as a Conservative, had been endorsed by some Republican luminaries, including Sarah Palin, the party’s 2008 vice-presidential nominee, and Gov. Tim Pawlenty of Minnesota, a likely candidate for president in 2012. The swell of opposition to Ms. Scozzafava was reflected on conservative radio talk shows and in a heavy diet of television ads supporting Mr. Hoffman that were financed by conservative groups.

Mr. Pawlenty said he was heartened that Ms. Scozzafava had stepped aside. “I’m glad Republicans are uniting behind a candidate who understands the federal government needs to quit spending so much,” he said.

Ms. Scozzafava did not say whom, if anyone, she would endorse. Polls showed that Mr. Owens and Mr. Hoffman were each drawing about 35 percent of the vote, while Ms. Scozzafava had around 20 percent. Several Republicans said that at least in theory, her withdrawal should help Mr. Hoffman as Republican voters join his campaign.

Mr. Hoffman, who appeared before a crowd of about 100 cheering supporters on Saturday afternoon at a Veterans of Foreign Wars hall in Plattsburgh, said he believed victory was within his grasp. “We have to finish this race fighting like the underdog,” he said. “We have to unite the Republicans, unite the independents and the Democrats who have had enough.”

The district has been solidly Republican since the 19th century, and had been represented by John M. McHugh, who stepped down after Mr. Obama named him secretary of the Army.

“In recent days, polls have indicated that my chances of winning this election are not as strong as we would like them to be,” Ms. Scozzafava said in a statement. “The reality that I’ve come to accept is that in today’s political arena, you must be able to back up your message with money. And as I’ve been outspent on both sides, I’ve been unable to effectively address many of the charges that have been made about my record.”

Her departure is a clear victory for conservatives at a time when there has been a pitched battle among party leaders over whether Republicans needed to change their ideological appeal as part of an effort to recover from the losses of 2006 and 2008.