The AP reports:

Veteran Republican Sen. Arlen Specter of Pennsylvania disclosed plans Tuesday to switch parties, a move intended to boost his chances of winning re-election next year that also will push Democrats within one seat of a 60-vote filibuster-resistant majority.

“I now find my political philosophy more in line with Democrats than Republicans,” Specter said in a statement posted on a Web site devoted to Pennsylvania politics and confirmed by his office. Several Senate officials said a formal announcement was expected later in the day or Wednesday.

President Barack Obama called Specter almost immediately after he was informed of the decision to say the Democratic Party was “thrilled to have you,” according to a White House official. Spurned Republicans said his defection was motivated by ambition, not principle.

This is probably an acknowledgment on Specter’s part that he was heading for a drubbing in 2010 in the Republican primary.

As Phil Klein of the American Spectator points out, in 2004 Specter beat conservative Pat Toomey by just 17,000 votes “in a bitterly fought primary in which [Toomey] was outspent 4 to 1.”