President George W Bush is coming under increasing pressure to withdraw his nomination of outspoken conservative John Bolton for UN ambassador.
The White House still hopes Mr Bolton can be approved
The Senate Foreign Relations Committee has refused to support him, and referred his nomination to the full Senate without a recommendation.
Mr Bolton has been plagued by charges of bullying and arrogance, which the White House say are unfounded.
A key Republican senator on the panel said he was the wrong man for the job.
George Voinovich still joined the other nine Republicans in sending the nomination forward, but said he would lobby fellow senators on the matter.
"It is my opinion that John Bolton is the poster child of what someone in the diplomatic corps should not be," he said.
"The United States can do better than John Bolton."
On Thursday the panel voted 10-8 along party lines to forward the nomination without endorsing it.
The BBC's Justin Webb in Washington says it is highly unusual for a committee where the president's party is in the majority to fail to support one of his nominations for office.
A leading Democrat senator, Joe Biden, told the BBC the time had come for Mr Bush to think again about his choice.
Others vowed to keep up their case.
"If this comes to the floor, we're going to have a fight," Senator Barbara Boxer said during the committee meeting.
The White House said it was confident Mr Bolton's nomination would be confirmed by the Senate, where Republicans hold a 55 to 45 majority.
US Secretary of State Condoleezza Rice reiterated her belief that he "is the right man for this challenging assignment".
If the White House does not withdraw Mr Bolton's name, the Democrats have two options open to them.
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"
They could try to persuade as many Republicans as possible to oppose Mr Bolton in the full Senate vote and thus embarrass the White House with the closeness of the result.
Or they could try to mount a filibuster, talking the nomination out and forcing the White House to back down.
The BBC's Matt Frei in Washington says that even if Mr Bolton's candidacy is confirmed, so many unpleasant details have emerged during the hearings that he would arrive at the UN as damaged goods.
The committee's vote on Mr 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.
Mr Bush has called Mr Bolton "a blunt guy" who "can get the job done at the United Nations".