"The world has been turned upside down," one Republican senator said after Jeffords revealed his decision Thursday.
The bombshell defection marks the first time in U.S. history that control of the Senate will change hands without an election, and gives the Democrats a Senate majority for the first time since 1994.
It also means a much tougher battle for President Bush's conservative agenda from a Democratic-led Senate eager to challenge him on matters from oil production and judicial nominations to abortion rights and education funding.
"I do not take this question lightly," an emotional Jeffords told a cheering crowd Thursday in Burlington, Vt.
Jeffords, who has often been at odds with his party in a 26-year congressional career, cited differences on many of the key issues at the heart of the U.S. political debate since Mr. Bush came to office in January.
"Looking ahead, I can see more and more instances where I will disagree with the president on very fundamental issues: the issues of choice, the direction of the judiciary, tax and spending decisions, missile defense, energy and the environment."
Jeffords later expanded on the reasons for his momentous decision, telling reporters that he felt the president's budget provided insufficient money for education.
He said he told President Bush during a White House meeting on Tuesday that the administration's proposal for school spending would make Mr. Bush a "one-term president" and jeopardize GOP senators seeking re-election.
White House Chief of Staff Andrew Card said President Bush should have known sooner that Sen. James Jeffords was about to quit the Republican Party.
"I wish I had serve the president better," Card said Friday.
At the same time, Card and other senior White House advisers sought to deflect blame for Jeffords' departure. They put the onus on Jeffords, some using less-than-flattering terms.
In a telephone interview from Capitol Hill, the chief of staff said he had heard rumors that Jeffords might switch but did not take them seriously until Sen. Olympia Snowe, R-Maine, told him early Tuesday morning that she thought Jeffords was leaving.
Snowe, a close friend of Jeffords, had left Card a telephone message late Monday night.
As soon as they hung up, Card said he told the president and Vice President Dick Cheney, then arranged for Jeffords to meet separately with both men. Card said of the president, "He was incredulous."
Senate Democratic leader Tom Daschle, in line to become the chamber's new majority leader, spoke to the president on Thursday, having what the White House said was a "cordial conversation."
Their exchanges may soon get heated when Daschle brings to the Senate floor such long-stalled legislation as measures that would provide a patients' bill of rights and increase the minimum wage.
"The only way we can accomplish our agenda, the only way that the administration will be able to accomplish their agenda, is if we truly work together in, now, I guess, what we would call a tripartisan manner," Daschle said.
The new Democratic-led Senate will slow down and prompt big changes to some major Bush initiatives, like developing a missile defense system and expanding oil and gas exploration, and hinder his efforts to appoint more conservative judges to U.S. courts.
Mr. Bush's opponent in the Republican presidential primary, Arizona Sen. John McCain, struck out at his party, criticizing members for targeting Jeffords.
"For his votes of conscience, he was unfairly targeted for abuse, usually anonymously, by short-sighted party operatives from their comfortable perches in K Street offices (in Washington) and by some Republican members of Congress and their staff," McCain said in a statement.
The news rippled through Congress, with hundreds of lawmakers and staffers adjusting to the announcement. Jeffords said his decision would take effect by early next month, once Congress completed work on Mr. Bush's $1.35 trillion tax plan.
"Everything will be moderated and moved to the middle," Sen. Charles Schumer, a New York Democrat, said.
Sen. Trent Lott of Mississippi, who will lose his position as Senate majority leader, put on a brave face.
"There are some things that you do differently when you're in the minority, but I learned a long time ago you look at every circumstance as an opportunity, and we're going to continue to do that," he told reporters.
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