Sen. Arlen Specter’s April 28 announcement that he has switched from the GOP to the Democratic Party will give Democrats a filibuster-proof 60-vote Senate majority, if Al Franken retains his tight lead in the unsettled Minnesota race.

Specter, of Pennsylvania, will run as a Democrat in 2010. He was facing an uphill battle in the 2010 GOP primary against conservative Rep. Pat Toomey (R-Pa.).

Specter says the Republican Party has shifted far to the right of where it was when he was elected in 1980. “I now find my political philosophy more in line with Democrats than Republicans,” he said. But he also said he will continue to toe a moderate line and will not be an “automatic 60th vote for cloture.” For example, Specter said he still plans to oppose ending debate on the Employee Free Choice Act, a bill that would make it easier for unions to organize.

Senate Democrats such as Majority Leader Harry Reid (Nev.) and Robert Byrd (W.Va.) welcomed Specter’s announcement. But critics charge his party switch is more about election politics than political ideology.

“I don’t think it changes much,” says Stephen Sandherr, the Associated General Contractors’ CEO, noting that Specter typically votes independently of party affiliation. “He just bought himself a place in the Democratic primary” for 2010, Sandherr says.