By Justin Webb
BBC News, Washington
President Bush's UN ambassador is resigning his post because he cannot get Senate backing to stay in the job.
Bolton was appointed on a temporary basis
If Mr Bush's Republican Party had kept control of the Senate in last month's elections, they might have been able to get John Bolton confirmed.
But those Democrats who oppose him have the wind in their sails and they made it clear that the nomination was dead.
Mr Bolton's departure was announced by the White House on Monday morning - to the enormous satisfaction of those who believed he was too acerbic and too focused on American power to be a good ambassador for the nation.
On hearing the news, the UN Secretary General Kofi Annan chose his words carefully - but did not seem to suggest he had lost a friend
"I think Ambassador Bolton did the job he was expected to
do," was all he could manage.
But although Mr Annan was plainly unconverted there was equally plainly a case to be made that Mr Bolton should stay.
President Bush said he accepted Mr Bolton's resignation with deep regret.
That regret will be shared by many Republicans who see this as the culmination of a vindictive and personal campaign against a man who was widely acknowledged to be doing a good job at the UN.
The Chinese ambassador - with whom Mr Bolton worked closely on North Korea - said he had been hard-working and effective, and expressed regret that he was leaving.
Mr Bolton does seem to have impressed those who feel that the UN is too flaccid and politicised an organisation.
But he was also working for a president whose political power has been seeping away: He is a casualty of the wider weakness of the Bush White House.