President George W Bush has suffered a setback in his efforts to appoint an outspoken conservative, John Bolton, as US ambassador to the United Nations.
Accusations that Bolton is a poor manager have been denied
The Senate Foreign Relations Committee declined to give its backing to Mr Bolton, who has been plagued by charges of bullying and arrogance.
A key Republican senator said Mr Bolton was the wrong man for the job.
The move raises the possibility that opponents may block Mr Bush's choice, correspondents say.
Mr Bolton needed every Republican vote on the committee if his nomination was to be sent to the Senate with a recommendation, which would improve its chances of being passed by the upper house.
Republican senator George Voinovich said he would not endorse the choice.
Mr Bolton is "the poster child of what someone in the diplomatic corps should not be," said the Ohio politician.
But Mr Voinovich said the nomination should be considered by the full Senate - where Republicans hold a 55-44 majority - adding that he hoped it would be rejected.
"I am not so arrogant to think that I should impose my judgement and perspective of the US position in the world community on the rest of my colleagues," he told the committee.
"We owe it to the president to give Mr Bolton an up or down vote on the floor."
The White House said it was confident Mr Bolton's nomination would be confirmed by the Senate.
US Secretary of State Condoleezza Rice reiterated her belief that he "is the right man for this challenging assignment".
Yale Law School graduate
As assistant secretary of state under George Bush senior, helped organise anti-Saddam alliance
Made under-secretary of state for arms control and international security in May 2001
In July 2003, condemned North Korean leader Kim Jong-il for living like royalty while people lived in "hellish nightmare"
But this rare move by the committee is a slap in the face for a president who was re-elected just over six months ago, says the BBC's Matt Frei in Washington.
Even if Mr Bolton's nomination is confirmed, he is likely to arrive at the UN as damaged goods, our correspondent adds.
The committee's vote on John Bolton was suspended last month after allegations about his past conduct.
Mr Bolton - a well-known critic of the UN - was accused of bullying junior staff and trying to distort intelligence to fit his own views, but the White House said the charges were unfounded.
Ahead of the decision on Thursday, Mr Bolton reportedly stated his right to an independent view.
In a statement seen by the New York Times newspaper, Mr Bolton told the panel a policy maker should maintain the right to "state his own reading of the intelligence", even when it differed from that of intelligence agencies.
Mr Bush has called Mr Bolton "a blunt guy" who "can get the job done at the United Nations".