"I will leave the Republican party, and become an independent," he said, adding he would caucus with the Democrats. "Control of the U.S. Senate will be changed by my decision."
The bombshell defection marked the first time in U.S. history that control of the Senate would change hands without an election. With Jeffords as an independent, Democrats will become the majority party, with 50 seats to the GOP's 49.
"I do not take this question lightly," an emotional Jeffords said. Before a cheering crowd, Jeffords said he had no intention of quitting the party when he stood for re-election in the fall. In fact, in a pantheon of intriguing Senate races last year, Jeffords' bid for re-election in the Green Mountain State was hardly noticed.
One implication to be drawn was that Jeffords had come to his decision recently as a result of his inability to find common ground with the Bush administration. But Jeffords did not criticize Mr. Bush directly, citing "fundamental disagreements" with the increasingly conservative GOP and its leadership.
"Vermont has always been known for its independence and its social conscience," said Jeffords, speaking from the Radisson Hotel in Burlington, Vermont's largest city.
Jeffords' decision alters the landscape of Capitol Hill from issues of office space, to the Senate committees, to those of oil drilling in Alaska's wilderness, and beyond.
"It will absolutely devastate the president's agenda," said Republican consultant Eddie Mahe of Washington. "I'm stunned."
Jeffords did offer on olive branch to the Bush administration, saying his move will take effect after the signing of the tax bill, which made it through the Senate on Wednesday.
Empty judgeships could remain empty a little longer. Phone companies could find it harder to loosen federal regulation. President Bush's appointments, already slowed by the long-in-limbo election, could be further delayed.
Republicans across the country could find it harder to pump up enthusiasm.
"This has a terribly demoralizing effect on the Republican grass roots," said GOP consultant Scott Reed. "They will really scratch their heads and wonder why this happened."
Democrats who relished the change and Republicans who tried to dissuade Jeffords understood this was a seminal moment for the Senate.
"It's not only just chairmanships. It's staff, the country, the presidency. We're not just talking about a singular moment," said Sen. Olympia Snowe, R-Maine, a moderate and close friend of Jeffords.
"This isn't about a single Senate seat," said Sen. Robert Torricelli, D-N.J. "It's about controlling the legislative agenda ... and it's about the federal judiciary. This is an enormous shift of influence in the federal government."
Sen. Dick Lugar, R-Ind., one of the GOP veterans who tried to keep Jeffords a Republican, said the Vermonter thought about his decision with a sense of history and urgency in mind.
GOP sources, speaking on condition of annymity, said top White House adviser Karen Hughes conducted a conference call with congressional GOP aides Wednesday, telling them the White House wouldn't be pointing fingers of blame, and she hoped they wouldn't either.
Jeffords' relations with the White House have been strained for weeks. He backed reductions in Bush's original 10-year, $1.6 trillion tax cut in favor of increasing federal support for education.
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