Quotes of the Day

It was only a joke. In mid-February, during backroom negotiations over the economic-stimulus package, Nebraska Senator Ben Nelson tried to cut the tension. He turned to the three Republicans in the room and said, "If only you'd consider switching sides, then we'd all be on the same team and this would be a lot easier." Looking back on that moment, Democrat Nelson recalls something that seems far more telling now than it did at the time: Arlen Specter was the only one of them who didn't laugh.

Maybe the Pennsylvania Senator knew he was bound to wind up on the other side of the aisle eventually. Democrats — including Vice President Joe Biden, with whom he had shared many a ride on Amtrak when they were both commuting Senators — had been wooing him for years. Specter, 79, had been a Democrat until 1965. But when his latest turnabout finally happened, it caught the entire capital by surprise and altered everyone's calculation of what is now possible. Assuming that the interminable Minnesota recount battle finally ends with Al Franken being awarded the Senate seat — he holds a 312-vote lead — the Democrats will have a 60-vote Senate majority. That's the magic number it takes to beat back a Republican filibuster and, at least in theory, push through Barack Obama's big-ticket agenda items. Not since Jimmy Carter's days has a President's party had that kind of numerical leverage in the Senate. (Read about the top 10 political defections.)

In Carter's era, the Democratic caucus was riven by ideological differences and too disdainful of the President to work with him effectively. Senate associate historian Donald Ritchie says you have to go all the way back to the dawn of F.D.R.'s second term in 1937 to find a President aligned with a filibuster-proof Senate majority that has comparable cohesion and potential to pass significant legislation. "Doing the filibuster at every whim to block us is not [an option], and that makes legislating a lot easier," says New York Democrat Charles Schumer. (See a day-by-day look at the first 100 days of the Obama Administration)

Historic though the ratio may be, it does not give Obama an unfettered hand. On all sorts of issues, from health care to energy policy, Senate majority leader Harry Reid will still have to bring along his own right flank — moderate Democrats such as Nelson, Louisiana's Mary Landrieu, Indiana's Evan Bayh and now Specter. And not everything that is about to come before the Senate splits the Democrats along ideological lines. On climate change, for instance, the make-or-break votes come from a diverse group of 16 Democrats from left and center who say they will not support any bill that would impose crippling costs on the polluters who provide jobs for their constituents.

As for Specter, he has already declared that the majority to which he now belongs should not count him as an "automatic 60th vote." That's an understatement. Independent to the point of being exasperating, Specter was never a reliable Republican vote and isn't likely to be much more dependable for Democrats. He played a pivotal role in defeating the Supreme Court nomination of conservative icon Robert Bork in 1987 and famously invoked Scottish law to vote "not proved," therefore not guilty, in Bill Clinton's impeachment trial. Yet Democrats should not forget that he voted for George W. Bush's tax cuts and Supreme Court nominees and the Iraq invasion.

Specter's latest move — which he acknowledges came after seeing a private poll that showed he would be doomed in next year's GOP primary in Pennsylvania — appears to make him a much safer bet for re-election. The fact that he no longer has to worry about a challenge from the right changes the political equation on some issues. It means, for instance, that Republicans may be unable to filibuster Obama's judicial nominees. And Specter might be willing to reconsider his opposition to the controversial Employee Free Choice Act; his resistance to the measure, which would make it easier for unions to organize workers, all but killed it. While Specter said he would not support the bill in its current form, union officials are now far more hopeful that they can find a compromise that will bring him around.

Meanwhile, the shrinking party that Specter abandoned is left to ponder why its moderates are fleeing — not only in the Senate but also across the country. The latest Washington Post/ABC News poll shows only 21% of registered voters now identify themselves as Republican. There is always the possibility that Democrats will overreach. Senate minority leader Mitch McConnell warns, "There won't automatically be an ability to restrain the excess that is typically associated with big majorities and single-party rule." Obstructionism will not work anymore — which could mean it's time for Republicans to look for a governing agenda of their own.

See the world's most influential people in the 2009 TIME 100.

• Karen Tumulty and Jay Newton-Small / Washington