It toppled a longtime Democratic congressman from West Virginia on Tuesday, and several White House-favored lawmakers elsewhere are confronting liberal voters who don't want party elites telling them what to do.
The party controlling the White House typically loses seats in a president's first midterm elections, but anger and frustration with Washington is even more intense this year amid high unemployment and record home foreclosures. Polls show public approval of President Barack Obama and the Democrats has dropped and roughly half of Americans want to fire their own congressman.
Specter's nomination seemed virtually assured last year when the entire Democratic establishment, including Gov. Ed Rendell, backed him in exchange for his switch from the GOP. But Rep. Joe Sestak, who bills himself as the contest's true Democrat, has erased Specter's big lead in the polls.
Tuesday's vote is expected to be close, and Obama has cut a last-minute TV ad for Specter in hopes of avoiding an embarrassing upset.
Should Specter lose, he would be the third prominent politician in a month to fall in intraparty contests dominated by restless voters who show little respect for well-established figures and party leaders.
In Florida, Republican Gov. Charlie Crist was expected to cruise into the Senate, with barely a thought to his party's primary. But conservative Marco Rubio and tea party activists drove Crist out of the GOP, and he is running as an independent.
In Utah, 17-year Senate veteran Bob Bennett fell victim Saturday to the once-unthinkable claim that he's not conservative enough for the Republican Party. His sins, according to tea party activists who taunted him at a GOP convention, include voting for the 2008 bank bailout pushed by Republican President George W. Bush.
In West Virginia, 14-term Rep. Alan Mollohan lost the Democratic nomination for his seat to state Sen. Mike Oliverio on Tuesday.