The Democratic candidate for the Massachusetts 10th Congressional District, Bill Keating, right, and his Republican opponent, Jeff Perry, sit together Oct. 13 while waiting on the set for their televised debate to begin at the New England Cable News network studio in Newton, Mass. In the congressional district that’s home to the Kennedy compound, the heart of liberalism is skipping a beat.

Hyannis Port, Mass. ? In the congressional district that’s home to the Kennedy family compound, a Kennedy public skating rink and a Kennedy museum, the heart of liberalism is beating uneasily.

Republican Jeff Perry is making a serious bid to take over a seat held by Democrats for nearly 40 years — and it’s just one of nearly 100 seats across the country that now appear under at least some threat of slipping away from the majority party and giving control of the U.S. House to the GOP.

At least 75 House seats — the vast majority held by Democrats — are at serious risk of changing hands, and roughly 25 more where Democrats were assumed to have the upper hand have tightened in recent weeks, raising the possibility that some could flip to the Republicans as well.

Perry, a Massachusetts state representative, is in one of those contests here in the 10th Congressional District, which stretches from Quincy, just outside Boston, along Cape Cod and across to the vacation playgrounds of Martha’s Vineyard and Nantucket. He is talking tough on taxes, immigration reform and the health care law, and he’s locked in a competitive race with Democrat William Keating for the open seat.

It’s a surprising turn in Massachusetts and just one of nearly a dozen contests across the country illustrating trouble in Democratic paradise.

Republicans have long believed they have a chance to win back the House, and possibly the Senate. Now, emboldened by polls showing even more of their candidates running strongly, they’re reaching into territory where Democrats were thought to be safe, in races from New York to Georgia to Wisconsin and Arizona.

Even the longest-serving Democrat, Rep. John Dingell of Michigan, who has coasted to re-election in recent years, is mounting an aggressive campaign against his GOP challenger, Rob Steele. Amid whispers that Dingell could be in danger, the Democrats’ House campaign arm last month issued a memo saying he would win but adding pointedly: “All Democratic incumbents should take a page out of his playbook and ‘run scared’ each cycle, especially 2010, even if they are not truly afraid of losing.”

The Republicans’ strategy is part psychology and part raw numbers: By targeting lawmakers once viewed as safe, the GOP can goad Democrats into spreading their money and energy across more races. By placing more bets around the map, however long the odds, Republicans increase their chances of reaching the 40-seat gain that would drive out House Speaker Nancy Pelosi and hand them House control.

Emotions are high.

In Hyannis Port, someone swiped a pair of “Jeff Perry for Congress” signs from the front of chiropractor Kristin Weber’s heavily trafficked office, so she drove to her mother’s house on a side street and grabbed one to replace it.

“I think people are fed up with the government,” said Weber, 41. “They’re fed up with the overspending. They’re fed up with the bailouts. It has not helped the economy, whatsoever. And we really do need change we can believe in.”

Other voters also suggest the Democrats have reason for alarm.

“There’s so much dealing; power tends to corrupt,” said Philip Faulk, 57, a jewelry store owner from Thomasville, Ga. He lives in the district represented by a nine-term congressman, Sanford Bishop, who is running against state lawmaker and Baptist minister Mike Keown.

Faulk, an independent, says he leans Republican.

Dan Kapanke of Wisconsin, a Republican state senator challenging veteran Rep. Ron Kind, said: “It’s resonating out here that, ‘Who is Ron Kind representing?’ That’s a question I hear.”

Ruth McClung, a 28-year-old scientist working for a Tucson, Ariz., defense contractor, is similarly optimistic about her chances against Rep. Raul Grijalva, who coasted to easy re-election in 2006 and 2008.

Says McClung: “We have a lot of grass roots that are fighting for it. It’s politics, but I think we want it more than they do.”

Some of the more long-shot GOP challengers are being propelled by the tea party movement. McClung, for example, won movement support after Grijalva called for a boycott of Arizona when his home state enacted a tough immigration law that’s since been blocked by a federal judge.

“We about two months ago said that was a tactical, logistic mistake,” said the congressman. “It was bound to tighten up.”

Other Republicans are backed by nationally known surrogates such as Sarah Palin and Sen. Scott Brown, R-Mass., who won the special election to replace the late Sen. Edward M. Kennedy in January and, in the process, gave confidence to Republicans nationally.