"Senator Specter's decision today represents the height of political self-preservation."
Sen. John Cornyn, R-Texas, chairman of the National Republican Senatorial Committee
"I welcome Senator Specter and his moderate voice to our diverse caucus, and to continuing our open and honest debate about the best way to make life better for the American people."
Senate Majority Leader Harry Reid, D-Nev.
"Republicans look forward to beating Senator Specter in 2010, assuming the Democrats don't do it first."
Michael Steele, chairman of the Republican National Committee
"Ultimately, we're heading to having the smallest political tent in history, the way events have been unfolding. If the Republican Party fully intends to become a majority party in the future, it must move from the far right back toward the middle."
Sen. Olympia Snowe, R-Maine
"This is a big moment. When Jim Jeffords left the Republican Party, he was the canary in the coal mine warning of Bush era ideological excess, and Senator Specter's decision eight years later signals its tipping point."
Sen. John Kerry, D-Mass.
"I don't think this will result in big changes on policy ... It will, however, help us get to substantive debate on important issues."
Sen. Claire McCaskill, D-Mo.