Pennsylvania Sen. Arlen Specter, a five-term senator facing a tough Republican primary battle next year, announced today that he will officially switch parties in the Senate and run as a Democrat next year.

"Since my election in 1980, as part of the Reagan Big Tent, the Republican Party has moved far to the right," Specter said in a statement. "Last year, more than 200,000 Republicans in Pennsylvania changed their registration to become Democrats. I now find my political philosophy more in line with Democrats than Republicans."

Specter's decision would give Democrats 60 members in the Senate, assuming Al Franken is eventually seated after a prolonged election in Minnesota. Whether Democrats can count on Specter for every vote, though, is another story. He said in his statement today that he would not be a "party-line voter any more for the Democrats that I have been for the Republicans."

Specter is described this way in the most recent edition of The Almanac of American Politics: "He is respected by colleagues and constituents, though not always well-liked. He sides with conservatives on some divisive issues, with liberals on others, building up no permanent credit with either."

Vice President Biden, Pennsylvania Gov. Ed Rendell and Sen. Bob Casey (D-Pa.) had been lobbying Specter to switch parties, as had other Senate Democrats, including Majority Leader Harry Reid.

Specter now avoids a potentially career-ending primary and fares far better against Pat Toomey in a general election, as conservative Republican primary voters would likely choose his more-conservative challenger. Toomey came within 2 points of defeating Specter in a 2004 Senate primary, and he left his day job as head of the Club for Growth to challenge Specter again. Recent polls showed Specter trailing Toomey by significant margins.

Republican prospects in the state were already unsteady, as President Obama won Pennsylvania by 11 points in November. John Cornyn (R-Texas), head of the National Republican Senatorial Committee, wrote a letter earlier this month in support of Specter and said he "is our best bet to keep this Senate seat in the GOP column."

"My job as head of the NRSC is to guide the GOP back to a majority in the Senate," Cornyn wrote. "I can't do that without Arlen Specter."

UPDATE: Below are released statements from Reid and Republican National Committee Chairman Michael Steele.