Senator Arlen Specter of Pennsylvania has announced he’s leaving the Republican Party to join the Democratic caucus on Capitol Hill. The move could have a major effect on the balance of power in Washington. If Minnesota Senate winner Al Franken prevails in his legal battle with former Senator Norm Coleman, he and Specter would give the Democrats sixty seats, enough to overcome a Republican filibuster. On Tuesday, Specter said he is making the switch on ideological grounds.

Sen. Arlen Specter: “As the Republican Party has moved farther and farther to the right, I have found myself increasingly at odds with the Republican philosophy and more aligned with the philosophy of the Democratic Party. This is a painful decision. I know that I am disappointing many of my friends and colleagues. Frankly, I’ve been disappointed by some of the responses, so the disappointment runs in both directions.”

By joining the Democrats, Specter will avoid a tough challenge in the Republican primary ahead of the 2010 midterm elections. Despite switching sides, Specter says he’ll maintain his “independence.” On Tuesday, Specter vowed continued opposition to the Employee Free Choice Act, which would remove obstacles for workers to join unions. Democratic leaders say they will retool the measure to gain Specter’s support.