But three Republican sources said the congressman was also exasperated by the gridlock that has come to define Washington. One party leader pointed specifically to clashes between pragmatic Republicans and the unyielding tea party wing that holds sway in the House.
"Jon was frustrated - no secret," said Burlington County Republican Chairman Bill Layton. "For the last year he was dealing with people in the Republican Party down there giving him a hard time on the Sandy relief package, and then there was the closure of government."
Runyan, a former Eagles offensive lineman, had called the GOP brinkmanship during the federal government's shutdown "a failed strategy" and had openly opposed it. Layton called the 16-day shutdown the "final straw" in a Runyan decision that had long been building.
His move sets the stage for a contest in one of the few competitive House districts left in the country. Immediately after his announcement, three political handicapping reports moved the seat from "safe" or "likely" Republican - based on Runyan's previous success - to "toss-up."
"It goes from being not a very good Democratic pick-up opportunity to a great one," said Kyle Kondik, managing editor of Sabato's Crystal Ball at the University of Virginia.
Democrat Aimee Belgard, a Burlington County freeholder, quickly announced that she would run for the seat, while Layton said Republicans would work to avoid the kind of bloody primary fight that hurt them when they lost the seat in 2008.
Fresh off his reelection in 2012, Runyan saw his second term begin with the House's vote on a Sandy relief package - critical to his hard-hit district - stalled by Republican budget hawks who worried about wasteful spending.
"In some ways that was embarrassing, that Jon had to fight hard for something that should have just been a simple thing," Layton said.
More recently, after Runyan spoke out in opposition to the shutdown, his aides were flooded with vituperative e-mails from hard-liners across the country. One shutdown supporter paid an angry visit to a Runyan office.
Two New Jersey Republicans, who did not want to be named discussing Runyan's thinking, played down the role of any one political faction. They said he was more generally frustrated by the Capitol's regular stalemates.
"Politics down there doesn't seem to be much of a team sport," said one source.
Runyan, 39, cited different reasons in his official statement.
"Politics shouldn't be a career and I never intended to make it one," he said in a news release. He called it "an honor" to have served his district.
Runyan's children are 11, 14, and 16. His oldest, who plays football at St. Joseph's Preparatory School in Philadelphia, has committed to playing football at Runyan's alma mater, the University of Michigan, in 2015, and the congressman hopes to spend time watching him play.
Runyan had won two tough elections after ending his 14-year NFL career. He ousted the late John Adler, a Democratic incumbent, in 2010 and survived a 2012 race with President Obama atop the Democratic ticket.
Adler's 2008 win was a blip for a seat long held by the GOP.
But Democrats see hope in the fact that Obama twice won the district, stretching across the state from Burlington County in the west to Ocean County in the east.
"Jon was under tremendous pressure," said Bill Haney, founder of the Burlington-based West Jersey Tea Party.
Haney, who has sharply criticized Runyan for being insufficiently conservative, called him a "good, honest man" and pointed to the attacks from Democrats on one side and conservatives tugging at him from the right. "I don't know how he was able to not tell those people off."
Democrat Belgard, who had been considering a run, made it official after Runyan's announcement. She has a news conference scheduled for Thursday.
"With all the political discord in Washington right now, I think we need folks that are real leaders," the freeholder said in a telephone interview. Pointing to the recent shutdown she said, "We need to do better and we deserve better."
Belgard, 39, is a lawyer and former Edgewater Park councilwoman. She has support from national Democrats, who cited Runyan's decision as evidence that GOP fortunes are sinking among moderate voters.
Republicans said they were confident of maintaining their long success in the district. One big help: Democratic stronghold Cherry Hill was removed in the 2010 redistricting.
The GOP's congressional campaign arm swiftly derided Belgard as "a Barbara Buono-style candidate who will have to run from her own record as well as Obamacare," referring to the Democratic gubernatorial candidate who was just crushed by Gov. Christie.
Layton said he would huddle with Ocean County Republican Chairman George Gilmore and others as they vet potential candidates and try to avoid an intraparty fight.
"We've been through this a few times over the last couple years and as a party we've learned from our mistakes," Layton said. "George and I are going to be working to avoid primaries, and we don't want to do anything that would threaten our ability to win the congressional seat."
Many Republican names were floated immediately after Runyan's announcement, though Layton said a decision could take weeks.
Evesham Mayor Randy Brown, a former Democrat who switched parties, said he was considering a run. Another name often mentioned in connection with the seat is Al Leiter, a former Mets and Yankees pitcher - though he has never run.
State Sen. Diane Allen (R., Burlington), also long considered a potential congressional candidate, told the website PolitickerNJ she would not run.
Don Adams, head of the Philadelphia area's Independence Hall Tea Party, had backed Runyan, calling him the kind of Republican who could win the moderate district and "maintain some kind of defense against the Obama administration."
Adams said he was stunned by the decision, and is worried about the fate of a seat that now becomes the focus of an intense battle - and one that is already drawing national attention.
Staff writer David O'Reilly contributed to this article.