"These are some of the only districts Republicans still represent that are fairly marginal, that are not all that partisan one way or the other," said David Wasserman, Cook's U.S. House editor. "These are the districts where the fallout would be most severe."
Fitzpatrick's seat moved from "likely Republican" to "lean Republican," making his one of the GOP's 19 most endangered districts, according to Cook. Runyan's and LoBiondo's went from "solid Republican" to "likely Republican."
"I don't believe we've had LoBiondo in anything but 'solid Republican' in decades," Wasserman said, adding that the incumbent could face his toughest challenger in years.
That challenger has a name older voters may remember. Bill Hughes Jr., an attorney whose father held the seat for 20 years before he retired and was succeeded by LoBiondo in 1995, is set to announce his candidacy Thursday at the Atlantic City boardwalk.
"Things are broken in Washington and it's not working in South Jersey," Hughes said Wednesday in an interview. "After almost 20 years in Congress, we've seen the best Frank LoBiondo has to offer."
Republicans have readied their counterattack. They've bought more than a dozen Internet domain names in variants of "Bill Hughes Jr. for Congress." Each will direct visitors to a site slamming him.
In Runyan's district, Burlington County Freeholder Aimee Belgard has been talking to the national Democratic Congressional Campaign Committee (DCCC) about a potential run against the former Eagles offensive lineman, according to a Democratic source who spoke on condition of anonymity.
On the other side of the Delaware, Fitzpatrick already has two challengers - former Army Ranger Kevin Strouse (backed by national Democrats) and businesswoman Shaughnessy Naughton. They spent the shutdown hammering the incumbent.
"If ever there was a reason to evict Congressman Fitzpatrick, this was it," said one Naughton news release. A Strouse blast said, "The reckless behavior of Congressman Mike Fitzpatrick and the far right wing . . . emphasizes the need for a new approach."
Fitzpatrick, LoBiondo, and Runyan, though, all spoke against the shutdown and helped end the fight by crossing party lines to vote with Democrats, and against hard-line Republicans, to get a compromise bill through the House.
For some Republicans, that might have sparked a challenge from the right, but local tea party leaders said this week that they still support the region's GOP incumbents.
"There is no doubt that shutting down the government was a failed strategy, which is why I opposed it very early on," Runyan said Wednesday in a statement. "But instead of recognizing the work that Delaware Valley Republicans are doing to actually govern this country, political groups like the DCCC choose to level attacks."
Democrats argue the local Republicans, if they were serious about defying their leaders, could have helped end the shutdown sooner if they had supported procedural moves that might have advanced a spending bill faster.
Even with local Republicans breaking from their party on a key vote, though, the GOP's overall brand problems could be a drag on them.
The party's image hit an all-time low, as measured by a Washington Post-ABC News poll in a survey released Tuesday: 32 percent of the public saw the GOP favorably, 63 percent had an unfavorable view.
Democrats also took a hit, but it wasn't as large.
Runyan and LoBiondo hold two of just 17 Republican seats in districts that President Obama won last year, and the president came within decimal points of taking Fitzpatrick's district as well.
Despite Democrats' sensing an opening, their path to ousting any those three incumbents will be difficult.
Fitzpatrick and Runyan were both top targets a year ago, and each cruised to comfortable wins, even with Obama atop the Democratic ticket. LoBiondo has not had a close race in years.
Hughes, the Democrat who plans to take on LoBiondo, is a former federal prosecutor - but national Republicans are already attacking his more recent work as a defense lawyer, mining his firm's website for details about potentially embarrassing clients.
"The choice in this election will be very clear - a bipartisan leader and public servant like Congressman LoBiondo, or a criminal defense lawyer like Hughes Jr. who specialized in standing up for the rich, powerful, and criminally corrupt," said Ian Prior, a spokesman for the national Republicans' congressional campaign arm.
Advertising against any of the lawmakers in the Philadelphia media market will also be expensive, consuming Democratic resources that could go farther on cheaper airwaves elsewhere.
Most critically, for all of the anger at Republicans now, the election is still a year away. Democrats will keep reminding voters of the shutdown, but Republican operatives expect that as time goes on, frustration with Obama's Affordable Care Act will grow and become a true pocketbook issue, putting memories of the budget fight in the background.
Fitzpatrick, asked Wednesday about the challenge facing him, emphasized his work with Democrats on a bipartisan plan they pushed to end the shutdown (though it didn't gain any traction).
"Nobody won in this recent shutdown, most especially the American people," the Bucks congressman said. "It's most important we learn, move on, with a commitment to greater bipartisan solutions."
He also confirmed that this would be his last race for Congress. He had self-imposed a term limit.
Democrats hope to end his tenure even faster - and think the GOP may have just helped.