Forget Day 100—Day 99 was far more interesting. Shortly after noon on Tuesday, Sen. Arlen Specter announced that he will campaign for reelection in 2010 as a member of the Democratic Party. News of the longtime Pennsylvania Republican’s defection sent shock waves through the Capitol and left Democrats enthused and senior Republican Party officials scrambling for an explanation.

At a press conference in the Capitol today, Specter said this decision was motivated by the strain of “extreme” conservatism that has infused the Republican Party of late. “Since my election in 1980, as part of the Reagan Big Tent, the Republican Party has moved far to the right. … I now find my political philosophy more in line with Democrats than Republicans,” he said.

“The extremes have taken over,” he said to reporters later—pointing to the example of other fallen Republican moderates of the Bush era. “Sometimes a party asks too much.”

As the fifth-oldest member of Congress, Specter may have decided it was time to look out for himself. The extended Democratic primary between Barack Obama and Hillary Clinton registered 200,000 more Democrats than Republicans in Pennsylvania and—as with the rest of the country—the remaining state Republicans were more conservative and less likely to support Specter’s liberal positions like his support of abortion rights and stem cell research. The five-term senator only just slunk into office in 2004 with 52 percent of the vote and was polling 21 points behind GOP challenger Pat Toomey last week. Specter, who said he consulted these public opinion polls before making his decision, may just have been trying to keep his seat—any seat.

Having gleefully tossed all his GOP baggage overboard, Specter goes from being a moderate, unloved Republican at risk of being primaried out of public office in 2010, to the hero of the Democratic Party.

As with prior party defections in the Senate, from Strom Thurmond in 1964 to Jim Jeffords in 2002, this changes the Capitol’s mathematics significantly. Specter is now the 59th sitting senator affiliated with the Democratic Party. (Joseph Lieberman, an Independent from Connecticut, has pledged to caucus with the Democrats.) If and when Democratic candidate Al Franken is seated as a senator from Minnesota, this will push the Democratic caucus number to 61­—a filibuster-proof majority of the size not seen since Lyndon Johnson’s Great Society.

By many accounts, the Democrats pushed hard for this victory. Specter called Democratic Majority leader Harry Reid with his decision before he called his own party leader Mitch McConnell, around 6:00 p.m. Monday night. Vice President Joe Biden, along with Pennsylvania Gov. Ed Rendell, had pleaded with Specter to become a Democrat when visiting Philadelphia back in March. And moderate Democratic Sen. Mary Landrieu of Louisiana greeted Specter with a full-throated “welcome” as he exited the Capitol subway this afternoon. President Obama reportedly spoke with Specter seven minutes after he heard the news, and told him Democrats are "thrilled to have you."

It’s yet unclear what specific incentives he was offered to join the Democrats. (Specter said that he and Reid had agreed it would only be “fair” for him to assume the seniority of a five-term Democrat on the powerful Senate Appropriations Committee, among others.) Specter says he will “not be an automatic 60th vote.” But one thing is sure: The age of Obama just got a lot more progressive.