The US Senate Foreign Relations Committee is shortly due to vote on the nomination of John Bolton.
But attacks on his track record have raised fresh doubts over his ability to perform in the role.
According to the New York Times newspaper, Democratic Congressional officials have sought to delay the committee’s vote, outlining at least five allegations of what they called abusive behaviour by Mr Bolton.
It was alleged that Mr Bolton had sought the removal of three government officials from their posts during his service as an under-secretary of state.
In an added twist, the Washington Post newspaper has reported that Mr Bolton withheld sensitive information about Iran from former Secretary of State Colin Powell and current department chief Condoleezza Rice.
The committee’s Republican chairman, Richard Lugar, said concerns raised about Mr Bolton did not warrant his rejection as the president selected representatives to the UN.
Mr Lugar said that Mr Bolton had denied any impropriety following committee testimonies last week, and had urged all information relating to these events be made public.
Opposition Democrats are seeking to sway at least one Republican member of the committee.
Their attentions have been focused on moderate Republican Lincoln Chafee who has occasionally aired reservations about Mr Bolton.
But one other Republican, Chuck Hagel, has indicated he might rethink his vote for Mr Bolton after expressing concern over his alleged bullying of subordinate staff members.
“I have been troubled with more and more allegations, revelations, coming about his style, his method of operation,” Mr Hagel told the CNN news service.
According to the New York Times, one staffer believed to have clashed with Mr Bolton currently serves on Mr Hagel’s staff.
The case is among several reportedly being reviewed by the Senate committee ahead of the vote on Mr Bolton’s nomination.