President George W Bush's choice as ambassador to the United Nations, John Bolton, has said that the UN needs US leadership to get it back on track.
Bolton said the US wanted to work in "close partnership" with the UN
Mr Bolton told a Senate confirmation hearing that the UN needed to focus more on human rights violations and international terrorism.
Mr Bolton, an outspoken critic of the UN, came under fire from Democrats.
Senator Joseph Biden said sending Mr Bolton as envoy to the UN would be like "sending a bull into a china shop".
At least one Republican has signalled he might vote against Mr Bolton.
If all the Democrats on the Senate Foreign Relations Committee oppose him as well, that will be enough to hold up his appointment.
'Gone off track'
Mr Bolton told the committee that the Bush administration "views the UN as an important component in our diplomacy".
"If confirmed, I pledge to fulfil the president's vision of working in close partnership with the United Nations," he said.
But he was adamant that "for the UN to be effective, it requires US leadership". He added: "I deeply believe that."
He said the UN had at times "gone off track", and outlined his four priorities if he was given the job:
- Strengthening UN institutions
- Stemming the proliferation of weapons of mass destruction
- Supporting the global war on terror
- Addressing humanitarian crises
But he came under hostile questioning from Democrats on the committee.
'No respect for UN'
Much of the questioning focused on Mr Bolton's treatment of intelligence officials in the state department.
He was accused of trying to have at least one official reassigned after he clashed with them about the threat from Cuba three years ago.
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"
The issue is being used by Mr Bolton's enemies to try to portray him as an ideologue who does not listen to advice, says the BBC's Justin Webb in Washington.
Mr Bolton is also accused of having no respect for the UN.
He once said that if 10 of the 38 storeys of the UN building in New York were lost it would not make any difference.
When challenged on this, he said he had been trying to tell an audience of world federalists that "there's not a bureaucracy in the world that can't be made leaner and more efficient".
Senator Biden, the committee's top Democrat, said he was surprised Mr Bolton wanted the UN posting "given all the negative things you've said about it".
Mr Bolton is one of the toughest campaigners in the Bush administration for a foreign policy based on US power and catering to narrowly defined US interests, say correspondents.
He is a staunch defender of the US-led military occupation of Iraq, and as Washington's top arms control negotiator has taken a hard line over North Korea's nuclear programme.
Supporters of Mr Bolton say he is intellectually capable and achieves results.
His boss, Secretary of State Condoleezza Rice, said of his nomination: "We've asked John because he gets things done."
"He is a tough-minded diplomat. He has a strong record of success and he has a proven track record of effective multilateralism," she said when the nomination was first proposed.
Opening the confirmation hearing, committee chairman Richard Lugar acknowledged the controversy surrounding Mr Bolton.
"The nomination... has generated public debate on US policies toward the UN and on the degree to which the US should embrace multilateralism," he said.
The hearing is likely to continue for several days.