His tone was less genial as he blasted the Club for Growth and its willingness to work to oust Republican incumbents to "purify the party." He cited former Rhode Island senator Lincoln Chafee as an example of someone who had been forced into a damaging GOP primary, then lost the general election.
Andy Roth, director of government affairs for the Club for Growth, called Specter's criticism "hogwash." Saying the Republican Party stands for lower taxes and limited government, he added: "If a party takes in whoever it wants, it doesn't stand for anything."
The National Republican Trust PAC issued a statement taking credit for Specter's departure. "The integrity of the Republican Party has just gone up," Executive Director Scott Wheeler said. And the Republican congressional campaign committee put out a fundraising appeal citing Specter. "Good riddance," the e-mail's subject line read.
'A loss for our party'
Republicans have fallen on hard times in the Northeast, once the base of the moderate Yankee wing of the GOP. There were 11 Republican senators from Maryland to Maine in 1984. Now there are three. In Pennsylvania, since Specter last ran in 2004 the number of registered Republicans has dropped by 200,000; the number of Democrats has risen by 400,000.
In March, Specter told The Hill newspaper that he would remain a Republican because the United States "very desperately needs a two-party system." He said then, "I'm afraid that we're becoming a one-party system, with Republicans becoming just a regional party" with a Southern base.
David Girard-diCarlo, a Pennsylvania lawyer who co-chaired George W. Bush's 2004 re-election campaign in the state, says he shares Specter's frustrations.
"When you look at the demographics in the Northeast, we seem to be running out of Republicans, and that is a cause of concern," says Girard-diCarlo, who served last year as ambassador to Austria. "The Republican Party should take a hard look at itself. If we can't be a national party, then we have some serious introspection that I think must occur."
He called Specter's switch "a loss for our party to be candid," and adds: "He's going to be extremely hard to beat."
Specter said it wasn't clear whether endorsements by Obama and Rendell would be enough to clear the field in the Democratic primary. Democratic Congressman Joe Sestak, who hasn't ruled out a race, issued a caustic statement. "Arlen has made a decision to leave a race because he could not win against someone," he said. "What needs to be known is what he is running for."
When Vermont Sen. Jim Jeffords switched from Republican to independent in 2001, he gave control of the divided chamber to the Democrats and shook the Bush White House. Some said Specter's decision could prove to be just as consequential.
"Unless (Republican) Norm Coleman can pull out his race (in Minnesota) it means that there will be no check and balance on the Democratic control," said Sen. Susan Collins of Maine, a moderate Republican, calling Specter's switch "a disappointing development."
The news and speculation about its repercussions overshadowed even the swine flu outbreak as a topic in official Washington. "Specter to switch parties?" Sen. Claire McCaskill, D-Mo., said in a tweet on her Twitter account. "Wow."
Other senators who switched sides
There have been 21 U.S. senators since 1893, including Pennsylvania's Arlen Specter, who have switched political parties while in office. Most did so after becoming disenchanted with their party's policies. A look at seven recent party switchers.
Chart reported by Catalina Camia
Sources: U.S. Senate and USA TODAY research.
Contributing: Fredreka Schouten and Richard Wolf in Washington; Mimi Hall in Arnold, Mo.