By Matthew Davis
BBC News, Washington
Accusations that Bolton is a serial bully are being investigated
US President George W Bush's controversial nomination of John Bolton to be his man at the United Nations has hit more turbulence.
New allegations have surfaced about Mr Bolton's past conduct, while critics in Congress are weighing into an unusually personal and protracted nomination process.
Despite robust support from the White House, doubts are growing over whether Mr Bolton will be confirmed in the role - which had seemed almost certain a week ago.
One of the most damaging developments came from a former US ambassador to South Korea who told the Senate committee investigating Mr Bolton's past how the pair had two undiplomatic rows in 2003.
Thomas Hubbard told the BBC Mr Bolton had misrepresented his views over a controversial speech on North Korea in July 2003.
In the speech, Mr Bolton described North Korean leader Kim Jong-il as a "tyrannical dictator" and the country as a "hellish nightmare" for its people.
Pyongyang was incensed, labelling him "human scum" and refused to continue negotiating with him.
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"
Mr Hubbard said he had contacted the Senate Foreign Relations Committee to "set the record straight" after Mr Bolton told them he had cleared the speech with the "appropriate bureaucracy".
"I had told him to tone it down as it used highly incendiary language about the North Korean leader - he did not do that," he said.
The former diplomat also said he told investigators about an occasion when Mr Bolton had put the phone down on him after learning Mr Hubbard had not arranged for him to meet the president-elect of South Korea.
"He then cancelled a dinner that I was supposed to host for him," Mr Hubbard told the BBC.
'Too close to call'
Last week, senators had looked set to vote along party lines and send Mr Bolton's nomination to the full Senate.
But in a surprise move, the committee delayed the vote until 12 May to allow more time to investigate Mr Bolton's management style after several Republican senators expressed concerns.
Conventionally, senators give the president the benefit of the doubt over nominations to the executive branch or to his diplomatic team.
Mr Bolton has said he will only respond to allegations to the committee itself, but in the meantime he is being forced to weather the storm.
Even senior Republicans are publicly questioning the nomination.
Senator Arlen Specter told CNN the odds on Mr Bolton being confirmed were "too close to call".
Linda Chavez, a former official in the Reagan and elder George Bush administrations, was even more pessimistic, telling CBS: "If I were betting now, I would bet that he's not going to make it."
Former US Secretary of State Colin Powell has also waded into the controversy.
The Washington Post, citing Republican sources, said Mr Powell had described Mr Bolton as an intelligent, but problematic government official in conversations with two senators on the committee.
Mr Powell's spokeswoman Peggy Cifrino confirmed only that Mr Powell had returned calls from senators about Mr Bolton.
While Mr Bolton has angered liberals with prior attacks on the UN, critics accuse the Democrats of using the accusations over his personal approach as weapons in a political battle to thwart the nomination.
White House spokeswoman Dana Perino said: "People are demanding reform at the United Nations, and John Bolton is the right person to help bring about much needed changes.
"He is smart, passionate, blunt and occasionally gruff - those are qualities required for an agent of change to get things done."
In the UK, the Foreign Office denied reports that Foreign Secretary Jack Straw had complained to Mr Powell about Mr Bolton's handling of nuclear negotiations with Iran.
Newsweek magazine cited an unnamed administration official who said he heard the conversation at a meeting in London in November 2003.
The Foreign Office said Mr Straw had "no recollection whatsoever about the incident".
"We have worked very closely with John Bolton in the past - I think what we are seeing is a storm in a teacup," an official told the BBC.