Pennsylvania Sen. Arlen Specter announced Tuesday that he is switching parties and will run for the Democratic nomination in the 2010 primary, reacting to "irreconcilable" differences with GOP primary voters.

With his decision to swap his "R" for a "D," Specter leaves the GOP before it can leave him: A Rasmussen Reports poll released Friday showed the five-term senator trailing former Rep. Pat Toomey, R-Pa., by an astounding 21 percentage points ahead of next year's GOP primary. A Quinnipiac University poll released in March gave Toomey a 14-point advantage.

That same Quinnipiac poll showed a sky-high 71 percent of Democrats giving Specter a favorable performance review, along with an overall 45-31 approval rating among Pennsylvanians. Whether Democrats' admiration for Specter as a moderate Republican will translate into primary support as he becomes a right-leaning Democrat is unclear.

The state's closed primary system means that only registered Republicans participate in choosing their party's candidate; the fact that more than 200,000 Republicans switched registration for the 2008 elections only increased the GOP contest's rightward tilt. Specter, always a moderate Republican, had taken considerable heat from Pennsylvania conservatives for his support of the stimulus package, and he acknowledged as much in his statement Tuesday.

"I have traveled the state, talked to Republican leaders and officeholders and my supporters and I have carefully examined public opinion," he said. "It has become clear to me that the stimulus vote caused a schism which makes our differences irreconcilable."

Conservative protests aside, Specter's stimulus vote played well with a majority of Pennsylvanians: Voters in the Keystone State supported the stimulus package by a 54-to-37 margin (though an even larger majority did not believe the stimulus plan would help them personally), according to a Quinnipiac poll from February. The March Quinnipiac survey found that Keystone Democrats favored the stimulus bill 87 percent to 6 percent, while GOP voters disapproved, 70 percent to 25 percent.

With Specter's switch and Al Franken (D) poised to be seated as Minnesota's new senator, the Dems are on track to secure 60 seats and a filibuster-proof majority, a prospect for which they thought they would have to wait until next year's midterm elections.

The surprise switch recalls former Vermont Sen. Jim Jeffords, who left the GOP in 2001 to become an independent, handing Democrats control of the Senate. Specter was mindful of the historical corollary as well, but insisted in his statement that the "D" next to his name would not change his votes.

"My change in party affiliation does not mean that I will be a party-line voter any more for the Democrats [than] I have been for the Republicans," he said. "Unlike Senator Jeffords' switch, which changed party control, I will not be an automatic 60th vote for cloture. For example, my position on Employees Free Choice (card check) will not change."