John Bolton, the controversial US Ambassador to the United Nations, resigned yesterday after it became clear that President Bush was unable to muster support for his reappointment.

Mr Bush, who received Mr Bolton’s resignation letter on Friday, said he was “deeply disappointed” that a “handful” of senators had blocked the appointment. But the resignation reflected the new reality in Washington of a greatly weakened President who, with Democrats controlling the House and Senate, can no longer count on getting what he wants.

Mr Bolton is the second high-profile casualty of last month’s midterm elections after Donald Rumsfeld’s departure as Secretary of Defence. Confirmation hearings for his successor, Robert Gates, begin today.

Mr Bolton, a blunt, irascible hawk who earlier in his career expressed contempt for the UN, was unable to get Senate confirmation last year and was installed to the post by Mr Bush in August 2005 by means of a recess appointment, when Congress was not in session.

Recess appointments are temporary, and without formal confirmation this month Mr Bolton’s tenure at the UN will expire when the current session of Congress adjourns on January 3.

As recently as last week Mr Bush was adamant that Mr Bolton should remain at the UN despite the fact that he lacked the votes on the Senate Foreign Relations Committee to move the nomination to a full floor vote. The White House had been looking for yet another way around the impasse, one option being to appoint him as an “acting” Ambassador for another few months.

But Mr Bolton’s fate was sealed with the Democrats’ takeover of the Senate after last month’s midterm elections. Joe Biden, the incoming chairman of the Foreign Relations Committee, said that he saw “no point in considering Mr Bolton’s nomination again”. The final blow came after Lincoln Chaffee, a moderate Republican on the committee who lost his Rhode Island seat on November 7, said that he would not vote for Mr Bolton if the nomination was considered in the final weeks of this congressional session.

John Kerry, Mr Bush’s 2004 Democratic challenger, said that Mr Bolton’s departure could be a turning point for the Administration.

“With the Middle East on the verge of chaos and the nuclear threats from Iran and North Korea increasing, we need a United Nations Ambassador who has the full support of Congress and can help rally the international community to tackle the serious threats we face,” Mr Kerry said.

There was no word from the White House on who would be nominated to replace Mr Bolton, but the choice will say much about the tenor of Mr Bush’s diplomatic intentions during his last two years in office.

One possible contender is the Afghan-born Zalmay Khalilzad, Mr Bush’s Ambassador to Iraq. One of the Administration’s most skilled diplomats, there has been a growing expectation that after nearly 18 months in Baghdad Mr Khalilzad’s time in Iraq is coming to a close, particularly as his efforts to co-opt Iraq’s Sunni minority into the political process has had little success. Another possible replacement for Mr Bolton is George Mitchell, the former Democrat senator and Bill Clinton’s special envoy to Northern Ireland, who played an integral part in the 1998 Good Friday agreement.

Mr Bolton’s hardline, uncompromising conservatism meant that he was deemed unsuitable for the job by Democrats and some moderate Republicans, particularly with his 1994 comment that if the UN building in New York lost ten storeys “it wouldn’t make a bit of difference”.

The sayings of John Bolton ...

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