Delivering a powerful message of discontent, voters Tuesday swept out veteran Sen. Arlen Specter in Pennsylvania, nominated a “tea party” movement founder for a Senate seat in Kentucky and forced Sen. Blanche Lincoln into a runoff for the Democratic nomination in Arkansas.

Looking shell-shocked and speaking to fewer than 100 supporters in a half-empty ballroom, Specter delivered a brief speech in Philadelphia conceding the Democratic race to two-term Rep. Joe Sestak — marking the end of his 30-year Senate career. Specter was a Republican the whole time save the last year, when he switched parties in a failed bid to keep his seat.

“It has been a great privilege to serve the people of Pennsylvania,” he said, with teary eyes, surrounded by his wife, son and grandchildren.

In Kentucky, political novice Rand Paul delivered the tea party its biggest win since the movement’s emergence last year amid a backlash over bailouts and President Obama’s expansive agenda.

Although Paul’s victory had been telegraphed by his big lead in polls, it was nonetheless seismic in scale, marking a significant breakthrough for the loosely knit protest movement. Up to now, activists have mainly lent their support to candidates who shared their principles, as opposed to backing one of their own.

“I have a message,” Paul told supporters at a victory party in Bowling Green, “a message from the tea party, a message that is loud and clear and does not mince words: ‘We have come to take our government back.’ ”

In a fourth race watched closely for its national implications, Democrat Mark Critz defeated Republican Tim Burns in the fight to fill a Pennsylvania House seat vacated by the late Democratic Rep. John P. Murtha. Exultant Democrats seized on the win as a sign that they can withstand an expected Republican tide in November and keep control of the House.

But the outcome elsewhere could give pause to many longtime lawmakers.

Specter’s defeat was the third loss for a congressional incumbent in less than two weeks and was perhaps the most resonant. Unlike Republican Utah Sen. Robert F. Bennett, who was defeated at a convention of conservative activists, or Democratic West Virginia Rep. Alan B. Mollohan, who was shadowed by ethical problems, Specter was rejected by a statewide electorate that shrugged off the clout that came with his seniority.

“It’s a wake-up call for every incumbent, Republican, Democrat, or anybody around the country,” a disappointed Pennsylvania Gov. Ed Rendell said as he left Specter’s election-night gathering.

Although he enjoyed the backing of Obama, Specter was laboring under perhaps the heaviest burden of any candidate Tuesday, defending not just his long tenure in Washington but his admittedly self-interested party switch. Specter quit the Republicans — after casting a key vote in favor of Obama’s economic stimulus plan, outraging conservatives — because he said there was no longer a place for moderates like him.

He built his campaign around the power he wielded as one of the longest-serving members of Congress — and how that worked to Pennsylvania’s benefit. But Sestak portrayed Specter, 80, as interested in nothing more than self-preservation and made frequent, if indirect, references to his age and history of health problems, including two bouts of Hodgkin’s lymphoma and a brain tumor. “His time has come and gone,” Sestak said.

Casting her ballot Tuesday, Johnelle Duncan, 72, agreed. “We need new blood, new ideas,” the retired librarian said after voting for Sestak at Philadelphia’s Simon Community Center.

Sestak, a retired admiral, addressed chanting supporters Tuesday night at a military academy outside Philadelphia.

“This is what democracy looks like,” he shouted. “A win for the people over the establishment, over the status quo, even over Washington, D.C.”

Sestak, 58, will face former Republican Rep. Pat Toomey, 48, in November’s general election. Toomey narrowly lost to Specter in the 2004 Republican Senate primary and is seen as the early front-runner to claim the seat.

In Kentucky, Paul started far behind Secretary of State Trey Grayson, the handpicked candidate of Senate Republican Leader Mitch McConnell. McConnell, an architect of the Kentucky GOP, helped shove aside junior Sen. Jim Bunning, a fellow Republican, because he considered Grayson more electable.

But Paul, 47, surged past him, thanks to a plain-spoken style and platform rooted in the tea party movement’s small-government tenets: He favors term limits, a balanced-budget amendment, a requirement lawmakers read every word of legislation before it passes and a stipulation that laws spell out their constitutional underpinning.

“He’s not anyone’s lap dog,” said Dianna Knight, 62, a retired high school math teacher in Louisville, who said she supported Paul precisely because so many in the GOP leadership opposed him.

Grayson and his allies sought to portray Paul, a Bowling Green eye doctor, as something of a kook. They cited, among other things, his support for legalizing medical marijuana and the view that America’s interventionist foreign policy was at least partly to blame for the Sept. 11 terrorist attacks.

But the criticism served mainly to rally Paul supporters — many of whom backed his father for president in 2008 — and helped reinforce his image as a political outsider.

Analysts consider the fall campaign against Democratic Atty. Gen. Jack Conway a toss-up, suggesting Paul may not stand up well to scrutiny in a state where Democrats outnumber Republicans and voters are accustomed to their long-serving lawmakers bringing home Washington largesse.

In Arkansas, Lincoln faced a fierce challenge from Halter, who attacked her equivocal stance on healthcare — she supported the overhaul bill on initial passage, but opposed its final approval — and accused her of being too friendly to Wall Street and insufficiently supportive of organized labor. Outside groups, led by labor unions, poured millions of dollars into a negative ad campaign pummeling Lincoln.

Lincoln and Halter will face each other in a June 8 runoff for the chance to take on Republican Rep. John Boozman in the fall. Boozman starts as a strong favorite to win that contest.

mark.barabak@latimes.com

seema.mehta@latimes.com

James Oliphant of the Washington bureau contributed to this report.