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“For the people who are Republicans that just sit by and allow them to continue to dominate the party after they beat Chafee, cost us the Republican control of the Senate and cost us 34 federal judges, there ought to be a rebellion. There ought to be an uprising,” he said.

Chris Chocola, the new president of the club, said Mr. Specter showed “how unprincipled he is.” Mr. Chocola said the club will now redouble its efforts to defeat Mr. Specter.

Moderate Republicans expressed disappointment in Mr. Specter’s move.

“That’s not something I would ever consider doing,” said Sen. Susan Collins, Maine Republican. “But I do think that our party needs to make clear that centrists are welcome, and sometimes that message is not sent as clearly as it should be.”

The last time a party held a filibuster-proof majority was in 1977 and 1978, when Democrats held 61 seats.

Mr. Specter’s change could mark the second time this decade that Republicans have been undercut by a party switch. James M. Jeffords’ switch from Republican to independent in 2001 gave Democrats a majority in the chamber.

• Kara Rowland, Donald Lambro and Ralph Z. Hallow contributed to this report.