I feel as if a weight has been lifted from my shoulders

Jim Jeffords

The move effectively hands control of the Senate to the Democrats for the first time since 1994 and could derail President George W Bush's policy agenda.

In an announcement in his home state of Vermont, Mr Jeffords said that he was becoming an independent but would vote with the Democrats.

Mr Bush said later: "I respect Senator Jeffords ... but respectfully I couldn't disagree more" with his decision.

A spokeswoman for Mr Bush said the White House was "disappointed," but the president would "continue to work with Republicans and Democrats, as he has been doing, to get results for the American people".

Disagreement

Mr Jeffords, 67, said he took the decision because he felt that he no longer supported the policies put forward by the Republican Party.

"In order to best represent my state of Vermont, my own conscience and principles that I have stood for my whole life, I will leave the Republican Party and become an independent," he said.

"Looking ahead, I can see more and more instances in which I will disagree with the president on fundamental issues," he told the news conference.

He felt a weight had been lifted from his shoulders, he said.

His speech was accompanied by cheers and shouts of "Thank you Jim".

His relations with the White House have been particularly strained in recent weeks, after his decision to oppose Mr Bush's proposals for huge tax cuts.

But Mr Jeffords said he had promised the president to delay the switch until after Congress completes work on the cuts.

Negotiators of the lower House of Representatives and of the Senate are working on a compromise version that could be written and win approval as early as Friday.

Finely balanced

The defection swung the finely balanced upper house, where each party previously had 50 senators.

The loss of Mr Jeffords means that the Republicans can no longer rely on Vice-President Dick Cheney's casting vote to push through legislation.

This will make it difficult for Mr Bush to get key points of his conservative programme through Congress, in the following areas:

• Plans to drill oil in sensitive areas such as the Arctic wildlife refuge in Alaska.

• Appointment of conservative judges to the Supreme Court to replace retiring members.

• The controversial education voucher system providing public funding for private education.

• The missile defence system.

• Confirmation of appointments to top positions in government. Only 11% have so far been approved.

Unconditional loyalty

Both Mr Bush and Mr Cheney met Mr Jeffords on Tuesday, urging him to remain a Republican.

But the New York Times newspaper quoted Republicans close to the administration as saying that the White House did not take rumours of Mr Jeffords' defection seriously until Monday evening.

The party leadership had expected unconditional loyalty since Mr Bush took office, the sources said, adding that the administration had tried to punish Mr Jeffords for his rebelliousness by cutting him out of important decision-making.

In the past 20 years, 14 Democrats in Congress have switched to the Republican Party, while only one Republican has gone the other way.

The defection will enable the Democrats to take control of some key committees.