Even so, Specter's switch gives him visibility and clout and "strengthens the hand of mainstream, pragmatic Democrats," says Sen. Ron Wyden, D-Ore. That could be especially important as Congress debates health care this summer. The change also makes it easier for Democrats to portray the GOP as ideologically narrow and rigid.
Democrats now can count 59 votes in their Senate caucus. Democrat Al Franken, ahead in Minnesota's Senate recount, would give the party 60 votes if he survives court challenges.
As a minority in the House and without the votes to filibuster the Senate, Republicans would find it harder to block Democratic initiatives or even be heard.
Senate GOP leader McConnell of Kentucky said the shift in power posed "a threat to the country" and questioned whether "our people want the majority party to have whatever it wants, without restraint."
"The loss of Specter is small," said Republican pollster Frank Luntz. "The loss of that 41st seat is huge."
'At odds' with GOP philosophy
At a jammed news conference on Capitol Hill, Specter was alternately defiant and jovial, ripping into conservative groups that had bedeviled him and joking with reporters. He talked with passion about his desire to continue to seek funding for medical research, particularly for the National Institutes of Health.
Democrats had been urging him to switch sides for five years, he said, but he described his vote in February for the administration's $787 billion stimulus package as creating a new "schism" with the GOP. He was one of three Republican senators to support it, earning criticism from radio talk-show host Rush Limbaugh and other conservatives.
Last Friday, Specter said he saw private campaign polls that showed him likely to lose the Republican nomination next year to Toomey, who came close to beating him in the 2004 primary and then led the conservative Club for Growth. Specter said he was "not prepared" to see his Senate career end by the judgment of "that jury," describing the GOP primary vote as dominated by hard-line conservatives.
"I have found myself increasingly at odds with the Republican philosophy and more in line with the philosophy of the Democratic Party," he said. Over the weekend, he decided to switch sides, first informing Reid and then McConnell about 6 p.m. Monday.
Specter has long been a survivor, facing near-death experiences in his personal health and his political career. Born in Wichita to Russian Jewish emigres, he was a Democrat until he weighed a race for district attorney in Philadelphia. When he switched to the GOP, it was "almost like changing my religion," he said in his biography, Passion for Truth.
Known for political independence and a prosecutorial style, he did not always win re-election easily. Even so, he is the state's longest-serving senator in history.
At midday Tuesday, when the news had broken, Specter walked through a crowd of reporters to the weekly luncheon of Republican senators, where he stayed for about 10 minutes.
Specter described the session as polite. "Sen. (Thad) Cochran said at least he wouldn't have to go to Erie any more to campaign for me," Specter said jokingly of his Mississippi colleague.