Mr.Davis

Mr. Davis, a wealthy industrialist, spent over $5 million of his own money on failed efforts to win a seat in Congress. He once said, after a congressman sent sexually suggestive messages to young pages, that he would have used a bat on anyone who had done the same thing to one of his sons. He also predicted that states with large numbers of Mexican immigrants would secede and start a second Civil War.

But these days, few are writing off Mr. Davis.

A special election fight over a House of Representatives seat, which has attracted big money and huge interest from both parties, now appears to hinge on him. Running as a Tea Party candidate, Mr. Davis, 78, is siphoning support from the Republican candidate — so much so that Republicans privately concede that if they do not stop him, they could hand a seat they have long held to the Democrats.

With a week remaining in the race, Republican heavyweights like House Speaker John A. Boehner of Ohio and conservative organizations, including one with ties to Karl Rove, are questioning Mr. Davis's conservative credentials in television advertisements, news conferences and elsewhere.

Mr. Davis's ascension comes as a race that had once seemed a certain Republican victory has become fiercely competitive because of a House Republican plan that calls for overhauling Medicare.

The election, in the 26th Congressional District, which runs between Rochester and Buffalo, is to fill a seat vacated by a Republican.

Mr. Boehner took a veiled swipe at Mr. Davis during a visit to western New York to campaign with the Republican candidate, Jane Corwin. Then, several national conservative organizations began directing attacks at Mr. Davis, who has switched parties multiple times.

In recent days, national and local Republican operatives have circulated a video that they claim shows Mr. Davis assaulting a young Republican volunteer who tracked him with a camera — an accusation the Davis camp says is untrue and a smear attempt.

Mr. Davis says Republicans are obviously out to get him.

"They're coming after me like I'm really the No. 1 issue to their winning the election," he said after a rally in Varysburg. "They thought this would be a coronation. And I've changed their plans, and they don't like it."

For the new Republican majority in Congress, the stakes in the race are significant.

Ms. Corwin's advantage vanished after her Democratic rival, Kathy Hochul, used the Republican Medicare proposal to energize her once long-shot campaign.

Ms. Hochul has repeatedly criticized