A former US official has said President George W Bush's choice for US ambassador to the UN, John Bolton, is a bully who is unsuitable for the job.
Bolton said the US wanted to work in "close partnership" with the UN
Ex-state department official Carl Ford was testifying before the Senate Foreign Affairs Committee examining Mr Bolton's nomination.
Mr Bolton, a critic of the UN, has denied trying to have people who disagreed with him sacked.
Intelligence director nominee John Negroponte has also faced a hearing.
It is unusual for a nomination hearing to concentrate so intensely on an individual's alleged personal failings, but Democrats claim that Mr Bolton has shortcomings which genuinely interfere with his ability to do the UN job, says the BBC's Justin Webb in Washington.
But it is a fight the Democrats will probably lose and Mr Bolton is still likely to be confirmed as America's UN ambassador, he says.
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 Ford used to run the state department's intelligence arm.
He said Mr Bolton was a serial abuser of junior staff, and that his treatment of an intelligence analyst who disagreed with him on the subject of Cuba damaged the work of the department.
"The collateral damage and the personal hurt that he causes is not worth the price that had to be paid. It simply is out of bounds for the federal bureaucracy to allow a bully to run wild over people," he told the Senate committee.
He described Mr Bolton as a "kiss up, kick down" bureaucrat.
This has been an extremely bruising process for Mr Bolton and he has been roughed up in the process in a manner which has surprised many observers, our correspondent says.
Democrats oppose Mr Bolton's nomination, but they need the support of at least one Republican on the committee to block it.
Rhode Island Senator Lincoln Chafee had been considering voting against Mr Bolton, but said he was impressed with his opening statement.
Mr Bolton's dislike of the UN as an institution has not been the central focus of the hearing.
He has said the UN needed US leadership to get it back on track, and should focus more on human rights violations and international terrorism.
In a separate hearing, Mr Bush's choice for new Director of National Intelligence, John Negroponte, has said his priority will be to make fundamental reforms to the US intelligence community.
He said he had built on the lessons learned about the shortcomings of the intelligence system both before and after the attacks on 11 September 2001.
Mr Negroponte, 65, a veteran diplomat, faced tough questioning by the Senate Intelligence Committee about his time as ambassador to Honduras, where human rights groups allege that he was aware of abuses by death squads.
He said he had done nothing improper and nothing which broke the laws applicable at the time.
The new post of national intelligence director was created on the recommendation of the commission that investigated the 11 September attacks.