But even before the event took place, Specter attended a Senate subcommittee hearing on the swine flu outbreak and took a seat on the Democratic side of the dais.
He made no overt mention of his decision, but said, "Sorry I can't stay longer, but this is a complicated day for me."
President Barack Obama called Specter almost immediately after he was informed of the switch to say the Democratic Party was "thrilled to have you," according to a White House official.
Spurned Republicans said his defection was motivated by ambition, not principle.
Republican National Committee Chairman Michael Steele said, "Let's be honest: Senator Specter didn't leave the GOP based on principles of any kind. He left to further his personal political interests because he knew that he was going to lose a Republican primary due to his left-wing voting record. Republicans look forward to beating Senator Specter in 2010, assuming the Democrats don't do it first."
Specter, 79 and in his fifth term, is one of a handful of Republican moderates remaining in Congress in a party now dominated by conservatives. Several officials said secret talks that preceded his decision reached into the White House, involving both Obama and Vice President Joseph Biden, a longtime colleague in the Senate. Pennsylvania Gov. Ed Rendell as well as Democratic leaders in Congress also were involved.
With Specter, Democrats would have 59 Senate seats. Democrat Al Franken is ahead in a marathon recount in Minnesota, and if he ultimately wins his race against Republican Norm Coleman, he would become the party's 60th vote. That is the number needed to overcome a filibuster.
Specter faced an extraordinarily difficult re-election challenge in his home state in 2010, having first to confront a challenge from his right in the Republican primary before pivoting to a general election campaign against a Democrat in a state that has trended increasingly Democratic in recent elections. Former Rep. Pat Toomey, whom Specter defeated in a close primary race in 2004, is expected to run again.
Specter has acknowledged in recent months that in order to win a sixth term, he would need the support of thousands of Pennsylvania Republicans who sided with Obama in last fall's presidential election.
"I am unwilling to have my twenty-nine year Senate record judged by the Pennsylvania Republican primary electorate," he said in the statement.
Asked by a reporter what he had to say to his constituents, Specter replied with a smile, "I don't have to say anything to them. They said it to me."
Specter has long been an independent Republican, and he proved it most recently when he became one of only three members of the GOP in Congress to vote for Obama's economic stimulus legislation. Then, he proved it once more, pivoting not long afterward to say he did not support legislation making it easier to form unions, a bill that is organized labor's top priority in the current Congress.
In Pennsylvania, the chairman of the state Republican Party, Rob Gleason, said that Specter should offer a refund to Republicans who have helped fatten his war chest, which totaled $5.8 million at the end of 2008. "He should give them the option," Gleason said.
Even before Gleason made his comments, Specter announced he would return donations he has received this election cycle "upon request."
Specter has long been one of the most durable politicians of either party in Pennsylvania. In recent years, he has battled Hodgkin's disease, a cancer of the lymphatic system, but maintains a busy schedule that includes daily games of squash.
As one of the most senior Republicans in the Senate, Specter held powerful positions on the Judiciary and Appropriations committees. It was not clear how Democrats would calculate his seniority in assigning committee perches.
As recently as late winter, he was asked by a reporter why he had not taken Democrats up on past offers to switch parties.
"Because I am a Republican," he said at the time.
"I welcome Sen. Specter and his moderate voice to our diverse caucus," Senate Majority Leader Harry Reid, D-Nev., said in a statement.
Slideshow archive | ABC13 wireless | Help solve crimes