Some Republicans reacted philosophically to the news of Specter's decision to leave the party. Sen. Lindsey Graham, R-S.C., a chief backer of Sen. John McCain's 2008 presidential campaign, said that while he was disappointed in Specter, the move should carry a lesson for Republicans.
"The situation in Pennsylvania highlights the dilemma facing the Republican Party," Graham said in a written statement. "Ideologically, we are a center-right party and I am committed to maintaining that position. However, for us to have national relevance we have to run and win in blue states. As a party we have to expand our base and diversify our membership while maintaining our fiscally conservative, limited government approach."
Sen. Harry Reid, D-Nev., welcomed Specter to the Democratic party, while stressing that bipartisan work will still be important in the Senate.
Saying that this is a time for the parties to reasses themselves, Reid added: "This is not a time to gloat or give high fives."
White House officials welcomed the news.
"I welcome my old friend to the Democratic Party. Senator Arlen Specter is a man of remarkable courage and integrity. I know he will remain a powerful and independent voice for Pennsylvania and the country," Vice President Joe Biden said in a statement.
President Obama called Specter shortly after receiving news this morning that Specter had switched parties.
"You have my full support," the president said, adding that he was "thrilled to have you."
Previously announced Senate candidate and former head of the National Constitution Center in Philadelphia, Democrat Joe Torsella, declared that he still intends to seek the Democratic nomination for Senate in Pennsylvania irrespective of Specter's announcement today.
"I decided to run for the United States Senate from Pennsylvania for one simple reason: I believe we need new leadership, new ideas, and new approaches in Washington. It's become obvious that the old ways of doing business might have worked for the special interests, but they haven't worked for the rest of us," he said in a statement. "Nothing about today's news regarding Senator Specter changes that, or my intention to run for the Democratic nomination to the Senate in 2010 -- an election that is still a full year away."
That could become a challenging run for Torsella, given the backing Specter is receiving from other Democrats, including Obama.
"If the president is asked to raise money for Sen. Specter, he is happy to do it," White House press secretary Robert Gibbs said at a briefing today. "If the president is asked to campaign for Sen. Specter, he is happy to do it."
ABC News' Huma Khan, David Chalian and Jake Tapper contributed to this report.