GOP Chair Michael Steele:

“Some in the Republican Party are happy about this. I am not. Let’s be honest-Senator Specter didn’t leave the GOP based on principles of any kind. He left to further his personal political interests because he knew that he was going to lose a Republican primary due to his left-wing voting record. Republicans look forward to beating Sen. Specter in 2010, assuming the Democrats don’t do it first.”

Sen. Chuck Schumer:

“Arlen Specter, through the years, has been an effective, intelligent and moderate senator. We welcome him into the Democratic Party and our caucus in the Senate with open arms and can understand that his party, particularly in the last three months, has shown no room for moderates. On a personal basis, I look forward to continuing to work with Arlen on a full range of issues. While it will still take a great deal of work to pass the President’s comprehensive and bold agenda, the Republican party will no longer be able to revert to kneejerk filibusters at every whim to block progress, and that is a very good thing for Americans.”

Former Pennsylvania Sen. Rick Santorum:

I spoke with Arlen this morning and he explained his reasoning to me. I told him I was deeply disappointed that he felt he had to do it. It is a huge blow to the Republican’s ability to moderate any of Obama’s very liberal proposals. I can only hope that Arlen will be as independent as a Democrat as he has been as a Republican.

President Obama (via Politico):

The president reached Specter, one of only three Republicans to support his stimulus package, on the phone at 10:32 and told him “you have my full support.” He added that we are “thrilled to have you.”