By Matthew Davis
BBC News, Washington
US President George W Bush has suffered an unexpected and embarrassing blow after a new delay in John Bolton's passage to becoming the US ambassador to the United Nations.
Although some Republicans had reservations about Mr Bolton, the Senate Foreign Relations Committee had looked set to vote along party lines and send his controversial nomination to the full Senate.
Accusations that Bolton is a serial bully are being investigated
But in a fractious meeting on Tuesday, one Republican senator sided with Democrats demanding a fresh hearing to air allegations that Mr Bolton was a "serial bully" who intimidated junior members of staff.
Aides are describing as "stunning" the latest development in an already protracted, and now highly personal nomination process.
Perhaps for the first time, it no longer seems certain that the fate of President Bush's choice for the job will run the way both of them would like.
Put woman 'through hell'
Mr Bolton's combative criticisms of the UN have endeared him to conservatives, but liberals and some moderate Republicans say he lacks the personal temperament for the UN job.
One of the claims against Mr Bolton is that he is a serial abuser of subordinates, and that his treatment of an intelligence analyst who disagreed with him on the subject of Cuba damaged the work of the state department.
Another dates back to 1994 when, working as a private lawyer, he is accused of putting a woman "through hell" when he represented a firm that was at odds with her client in a USAid project in Kyrgyzstan.
The White House describes the allegations as "unfounded".
And critics accuse the Democrats of using the accusations as a weapon in a political battle to thwart the nomination.
But what stunned many observers was that impassioned appeals against Mr Bolton in the committee hearing actually worked.
'Kind of refreshing'
Senator George Voinovich, a Republican from Ohio and not one of those who had previously questioned the nomination, said he had "heard enough today that gives me some real concern about Mr Bolton".
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"
When two other Republicans sided with Mr Voinovich, committee chairman Senator Richard Lugar had little choice but to ask staffers to pursue the allegations and reconvene in May.
Senator Lincoln Chafee, a Republican representing Rhode Island, did not make his views known at the hearing, but later told reporters he was glad the vote had been postponed.
"I don't know if I've ever seen, in a setting like this, a senator changing his mind as a result of what other senators said," he added.
"The process worked. It's kind of refreshing."
Pressure to quit
Mr Bolton - who has been a leading hawk on Iran, Iraq and North Korea - is seen as closely tied to the Bush administration.
The controversy over his nomination is more damaging than in the case of former New York City police chief Bernard Kerik.
Mr Kerik was President Bush's first pick for homeland security secretary but he withdrew last December after uncovering tax and immigration problems relating to his employment of a nanny.
While none of the Republican senators on the committee have said they will vote against Mr Bolton, the latest setback throws the nomination process into some confusion.
Republicans outnumber Democrats on the body by 10 to eight, but it is not yet clear what will happen in the event of a tied vote.
As the process drags on, the allegations against Mr Bolton will get more of a public airing.
Meanwhile, as the Bush administration stands by its man, there are growing calls for Mr Bolton to withdraw.
An editorial in the Los Angeles Times said: "President Bush's nomination of John Bolton to become United Nations ambassador began as an embarrassment and is ending as a disgrace."
It adds: "Bush may find it hard to back down, so Bolton should do him and his country a favour and step aside. Maybe there is a consolation prize the White House could offer him. How about ambassador to France?"