With about 43 percent of the state’s precincts reporting, Mrs. Lincoln had 44 percent of the vote to Mr. Halter’s 42 percent.
Still, Mrs. Lincoln’s campaign had sounded confident on Tuesday afternoon, issuing a statement declaring that she “has fought off this challenge from outside groups and ends the primary election period battle-tested.”
Mr. Halter jumped into the race just a few months ago after arguing that Mrs. Lincoln was betraying Democratic allies on labor issues and on health care reform.
Boosted by a wave of discontent among liberal voters, he got immediate support and campaign contributions from labor union members and pressure groups such as MoveOn.org’s political action committee, and his race shaped up as the key test for whether angry progressives could topple a sitting senator.
On the Republican side, Rep. John Boozman hovered right around the 50 percent mark needed to avoid a runoff in the crowded Republican field.
Even though the tea party adherents have received the most attention, liberal pressure groups showed they can flex electoral muscle as well.
For example, MoveOn.org’s political action committee backed Mr. Sestak, Mr. Halter and the winner of Kentucky’s Democratic Senate primary, Attorney General Jack Conway. Mr. Conway narrowly toppedLt. Gov. Daniel Mongiardo.
Already on Tuesday, Washington Democrats were touting the virtues of Mr. Sestak’s insurgent campaign.
“He knows what is wrong with Washington, and if elected to the Senate will shake up how business is done in the Capitol,” said Sen. Robert Menendez, New Jersey Democrat and chairman of the Democratic Senatorial Campaign Committee.
Democrats also gloated at Mr. Paul’s victory, calling it a rejection of Mr. McConnell, the Kentucky Republican who had endorsed Mr. Grayson. Democratic National Committee Chairman Tim Kaine said Mr. Paul is “outside of the political mainstream” and that Republicans have hurt their chances to keep the seat now held by retiring GOP Sen. Jim Bunning in November.
In Oregon, Sen. Ron Wyden was facing little opposition in gaining the Democratic nomination to run for a fourth term.
The electoral waters have already been bloodied this year by incumbents thrown overboard by voters.
Sen. Robert F. Bennett, Utah Republican, failed even to qualify for a two-candidate runoff as GOP primary voters punished him for working on a health care bill with Democrats — even though that measure failed and he voted against the eventual health care overhaul package.
In West Virginia, Rep. Alan B. Mollohan, a 14-term Democrat and member of the House Appropriations Committee, fell last week in the primary to conservative Democrat Mike Oliverio by more than 10 percentage points. Mr. Mollohan had been dogged by ethics accusations.
Democrats on Tuesday eagerly pointed to the special election for Mr. Murtha’s seat in Pennsylvania, which they called the evening’s most important race. They argued that if Republicans could not win that swing district they were unlikely to win enough seats in November to take control of the House.