With voters across the country embracing "outsiders" — from "tea party" candidate Rand Paul in Kentucky on the right to Pennsylvania Rep. Joe Sestak on the left — incumbents in both parties face a long, hot summer of trying to save their jobs.
From Sen. John McCain of Arizona to Rep. Charles B. Rangel of New York, high-profile lawmakers in both parties are girding to fend off the anti-establishment anger voters displayed in Tuesday's hotly contested primaries.
Mr. McCain is polling comfortably ahead of GOP primary opponent J.D. Hayworth, but Mr. McCain's staff shake-ups and some rightward policy shifts on issues including immigration indicate unease nonetheless.
Mr. Hayworth, a former congressman, said Wednesday that his campaign's tea party supporters will help him close the gap before the Aug. 24 primary, citing Mr. Paul's surprisingly large margin of victory in Kentucky as a sign of the movement's power.
In New York, Rep. Charles B. Rangel — dogged by charges of corruption — is entangled with serious Democratic primary challengers, including the campaign of a former aide looking to replace the 40-year House veteran.
Other incumbents expected to be in major fights include:
• Rep. Bob Inglis of South Carolina, who voted against the military surge in Iraq. He faces four opponents in the GOP primary on June 8.
• Rep. Mary Bono Mack of California. Like Mr. Inglis and Mr. McCain, she is facing a GOP primary opponent who has cast himself as "the real conservative" in the race.
• Rep. Jim Matheson of Utah, a five-term Democrat. He voted against President Obama's health care reform bill, prompting a challenge for the June 22 primary in conservative Utah.
• Sen. Michael Bennet of Colorado, a Democrat appointed to the seat 15 months ago, is being challenged by a former state House speaker.
Earlier this primary season, voters fired Sen. Robert F. Bennett, Utah Republican, and Rep. Alan B. Mollohan, West Virginia Democrat — and on Tuesday sent Sen. Arlen Specter in Pennsylvania packing.
"People just aren't very happy," Ira Robbins, 61, said in Allentown, Pa.
Several political analysts dubbed the special election for the seat long held by the now-deceased Democratic firebrand John P. Murtha in Pennsylvania an exception to the theme, and Democrats seized on it as an indicator that the projections of a Republican "wave" in November's midterm elections are overstated.
"The Republican Party's failure to take a seat that they themselves said was tailor-made for them to win is a significant blow and shows that while conventional wisdom holds that this will be a tough year for Democrats, the final chapter of this year's elections is far from written," Democratic National Committee Chairman Tim Kaine said Wednesday.
But Mark Critz, the Democrat who won the contest, ran a distinctly anti-Washington campaign himself, saying he opposed Mr. Obama's health care reform law — though he stopped short of supporting a repeal.
"There are seats [nationwide] where Democrats absolutely will be hurt by votes for health care or cap-and-trade [emissions trading programs] … so that was a smart set of positions for Critz to take," said John Fortier, a political specialist with the American Enterprise Institute, a conservative-leaning think tank. "It wouldn't surprise me that in a number of individual races you'll see that."
Several pundits suggested that Tuesday's results were more a rejection of Mr. Obama than of Washington or incumbents. Mr. Obama supported both Mr. Specter and Sen. Blanche Lincoln, a Democrat who was forced into a runoff in Arkansas by a challenger from the left, Lt. Gov. Bill Halter.
Those two will face off for the Democratic Party's Senate nomination on June 8 - and whoever wins then faces an uphill battle in the fall against the GOP nominee, Rep. John Boozman, who holds a decisive lead in the polls over either Democrat.
Other analysts warn of reading too much into Mr. Obama's support of the incumbent senators. Mr. Specter, who only last year bolted the Republican Party to become a Democrat, is seen as a turncoat by many Democratic voters. White House support for Mr. Specter was tepid, with the president deciding against visiting Pennsylvania in recent weeks on behalf of the incumbent.
"If I were the president, I wouldn't be too alarmed by what happened in Pennsylvania because you have a very attractive younger [challenger] who has been a Democrat all his life," said Karlyn Bowman of the American Enterprise Institute.
In Arkansas, a surprisingly strong show of support by organized labor helped push Mr. Halter into the runoff.
"And with Arkansas, a president almost has to endorse the incumbent," Mr. Fortier said.
White House officials said the president didn't watch the election results closely, but Tuesday's setbacks follow earlier losses for Obama-backed candidates in races in Virginia, Massachusetts and New Jersey.
But overall, presidential endorsements generally have little impact in swaying the results of congressional elections, many analysts say.
"Presidents are great at raising money for candidates, but they really don't have a lot of coattails when they go out and campaign for people, so I think it's a little hard to say you can pin these losses directly on the administration's doorstep," said Michael D. Tanner, a political analyst with the Cato Institute, a libertarian think tank.
Mr. Tanner added, however, that a major reason for so much nervousness among voters "is because the administration's policies have upset people."
"There's always a sort of anti-party-in-power dynamic [during midterms], but the level of anger here in both parties" is higher than usual, he said.
• Sean Lengell contributed to this article, which is based in part on wire service reports.