CHALLENGER Randy Altschuler, right, a Republican candidate in New York’s First Congressional District, with Dan Kirby, left, a campaign volunteer, and Tom Darrow, a campaign aide, going door-to-door in Shoreham, N.Y. Mr. Altschuler, a businessman, has poured his own money into his campaign.

RYE BROOK, N.Y. — New York has turned into a major battleground over the future of the House of Representatives, with Democrats forced to defend at least a half-dozen Congressional seats from unexpectedly strong Republican challenges.

Aided by third-party expenditures and President Obama’s sagging popularity, Republicans believe they are in a position to pick up seats in upstate and suburban swing districts that will help them retake control of the House.

In the Utica area, conservative groups and the National Republican Congressional Committee have unleashed a flurry of ads attacking Representative Michael A. Arcuri, a two-term Democrat trying to fend off Richard Hanna, a mall developer.

“It’s thermonuclear,” Mr. Arcuri said, referring to the ferocity of the campaign being waged against him. He added, “It’s going to be a close election.”

On Long Island, Representative Timothy H. Bishop, a four-term Democrat who has cruised to re-election in the past, faces an aggressive challenge from Randy Altschuler, a wealthy businessman who has poured his own money into his campaign.

In suburbs north of New York City, Representative John J. Hall, a two-term Democratic incumbent, has called on former President Bill Clinton for help in his fight against the Republican nominee, Dr. Nan Hayworth, an ophthalmologist and first-time candidate.

The Democrats’ problems here underscore the degree to which Republicans have expanded the map of competitive races around the country. In addition, Republicans see opportunity in New York because many of the seats now in play were won by Democrats during the past two election cycles.

“The fact that the Republicans are doing so well in New York suggests that Democrats are in trouble,” said Tory Mazzola, a spokesman for the National Republican Congressional Committee.

Republican strategists also said that the message that they are trying to hammer home — over big government programs advanced by Democrats, the economy and the federal deficit — resonates in the politically moderate districts that are their targets.

And while 60 percent of New York City residents approve of how President Obama is handling his job, his support is much lower in the suburbs around the city, according to a poll conducted by The New York Times last week. Forty-six percent of suburban voters approve of the president’s job performance while 40 percent disapprove.

The degree to which Democrats have been forced on the defensive can be seen in the 19th Congressional District, which Mr. Hall won in an upset in 2006 as Democrats took control of the House.

At a debate last week, the audience groaned loudly when Mr. Hall gave President Obama a grade of “B” for his job performance.

“He inherited a very difficult situation,” Mr. Hall said, later adding, “You don’t get credit for keeping a bad situation from getting worse.”

Mr. Hall reacted strongly when the debate moderator asked him if he would accept support from Representative Charles B. Rangel, the powerful Harlem Democrat who is facing a trial in the House next month on 13 ethics violations.

“I have not asked Charlie Rangel to come up here since his ethical difficulties surfaced,” he said before denouncing his rival, Ms. Hayworth, for accusing him of donating $1,000 to Mr. Rangel, the dean of the New York Congressional delegation. “I did not!”

As elsewhere in the country, Republican challengers in New York are getting support from numerous conservative advocacy groups pouring money into television advertisements.

They include Revere America, a group run by former Gov. George E. Pataki that is dedicated to repealing the health care law enacted earlier this year; the 60 Plus Association, which offers itself as the conservative alternative to AARP; and the Alliance for America’s Future, which is associated with former Vice President Dick Cheney’s daughter Mary Cheney.

The groups have spent about $1 million on advertisements in New York since the campaign season began heating up in September, according to strategists who are monitoring the races. Those attack ads have forced Democrats to spend time responding, instead of focusing on their Republican opponents.

Mr. Arcuri suggested the barrage of advertisements in the Utica area was taking a toll. “It is hard to keep a sizable lead when you are being outspent two to one,” he said.

But he indicated that meddling by outside groups in his district would likely backfire. “These are groups that have no presence or interest whatsoever in our district,” he said. “And I think people know that. And it is resented.”

On Long Island, Mr. Bishop’s campaign has dispatched volunteers to hand out literature countering claims by the 60 Plus group that medical coverage for the elderly will be cut by the new health care law, which Mr. Bishop supported. “Virtually every one of the claims they make are inaccurate,” said Mr. Bishop, who predicted the race would be close right up to Election Day. “It’s a tough race.”

Republicans also seem well positioned to pick up a seat in the state’s Southern Tier in the western part of the state that became vacant this year when the Democratic incumbent, Eric J. Massa, resigned after being accused of sexually harassing an aide.

In that race, a Siena College poll taken a month ago indicated that Tom Reed, a former Republican mayor of Corning, had a 14-point lead over the Democrat, Matt Zeller, an Army veteran who enlisted in the wake of the 9/11 terrorist attacks, but 26 percent were undecided.

The Democratic Congressional Campaign Committee appears resigned to Mr. Zeller’s loss, and has not devoted any spending to that race, though they are aggressively helping other Democrats who are targets. Republicans are also waging a strong challenge in the 23rd Congressional District in the northernmost region of upstate New York.

In the race there, a recent Siena poll showed the Democratic incumbent, Representative Bill Owens, had 46 percent, and the Republican, Matt Doheny, was supported by 39 percent. Mr. Doheny’s campaign got unexpected help this month when a third-party conservative candidate, Douglas L. Hoffman, withdrew from the race and threw his support behind the Republican.

Both parties are also closely watching a few other Democratic-held districts in New York that analysts say Republicans could capture if conditions deteriorate further for Democrats.

The districts include the 25th in the Syracuse area, where Representative Dan Maffei, a first-term Democrat, is facing Ann Marie Buerkle, an anti-abortion activist who picked up an endorsement from Sarah Palin.