Specter's switch from Republican to Democrat means that if Al Franken wins in Minnesota, Democrats would have 60 votes in the Senate, raising pressure on whoever loses a grueling court battle over the seat to extend the fight even longer.
Franken emerged from a statewide recount and court challenge with a 312-vote lead over Republican Norm Coleman, but Coleman has appealed to the Minnesota Supreme Court and the seat remains unfilled five months after Election Day.
Coleman has argued that the state wrongly rejected more than 4,000 absentee ballots, and his appeal is scheduled for oral arguments before the Minnesota Supreme Court on June 1.
Even if Coleman loses there, he could carry his appeal to the U.S. Supreme Court or start a new case in federal court.
Whether that would block Franken from being seated is unclear. The Senate requires him to have an election certificate to be seated, and Republican Gov. Tim Pawlenty - who signs the certificate - has hedged on whether he would do so if the case is being appealed beyond state courts.
Both campaigns issued careful comments on Specter's news.
"Senator Coleman's focus remains on the thousands of Minnesota citizens who have not had their voices heard or their votes counted," campaign manager Cullen Sheehan said. "We will keep on fighting to enfranchise these voters and to ensure that every legally cast ballot is opened and counted."
"Sen.-elect Franken looks forward to working with senators of both parties to make progress on President Obama's agenda and move our country forward," spokesman Andy Barr said.
Ross Baker, a political science professor at Rutgers University, said the pressure will be on Coleman to keep his appeal process going as long as possible.
"I don't think anybody fights as fiercely as when he's cornered," Baker said. "Coleman's now very much cornered, so I think his fangs will be bared. We'll see a resolute effort to stretch this thing out, in my opinion."
The race has already gone on longer than any election in Minnesota history and is about five months shy of being the longest contested U.S. Senate election in the modern era. A bitterly fought 1974 New Hampshire race didn't end until a new election was held in September 1975. But the state had a temporary senator in the interim - something Minnesota law doesn't permit.
Coleman and Franken were virtually deadlocked on Election Night, triggering an automatic recount of 2.9 million ballots. Coleman led by 215 votes heading into the recount, but Franken took a 225-vote lead after a comprehensive examination of ballots, including more than 900 absentee ballots that were ruled to have been improperly rejected.
Coleman appealed the results of the recount, but a three-month trial heard by a special three-judge panel actually ended with most of Coleman's claims rejected and Franken's lead growing slightly.
A statewide poll by the Minneapolis Star Tribune published Sunday found nearly two-thirds of respondents felt Coleman should concede the race.
John J. Pitney Jr., a professor of government at Claremont McKenna College in California, said Coleman is caught in a vise.
"There's certainly a lot of pressure on the home front because a lot of people in Minnesota don't think he has a case," Pitney said. "But Republicans are very eager to take any opportunity they can get at this point. From a Republican perspective a longshot is better than no shot at all."
Pawlenty on Tuesday rejected the idea that he would be under added pressure in deciding whether to sign the election certificate. He said he hadn't spoken to Coleman, his fellow Republican, about it.
"We're going to follow the law with respect to the Franken-Coleman litigation and when and how a certificate is issued," he said. "The situation with Pennsylvania has no connection or impact on what's going to happen in Minnesota."
Minnesota has been represented by a single senator, Democrat Amy Klobuchar, since Coleman's term expired in January.
With a win by Franken and Specter's switch, Democrats would have 60 votes in the Senate, including two independents who usually vote with them.
Under Senate rules, a single senator can object to consideration of a bill, in which case it takes a 60-vote supermajority to bring a bill to the floor or end debate so a final vote can be taken. Democrats were the last party to reach that critical mark in the 95th Congress of 1977-1979, when they held 61 seats against 38 Republicans and one independent.
By Associated Press Writers Brian Bakst and Henry C. Jackson