Republicans were looking for swift approval from the Foreign Relations Committee on Monday.
US President George Bush's selection of Bolton last month stirred controversy because of his expressed disdain for the United Nations and the blunt criticism he has levelled at arms control treaties and countries such as North Korea.
Bolton, 56, who served in the past three Republican administrations, has been one of his party's strongest
conservative voices on foreign affairs issues. He is now the administration's arms control chief.
Rice defends Bolton
Secretary of State Condoleezza Rice has come out in Bolton's defence saying "not everybody is given to subtlety and indirection". She said Bolton is a good negotiator and would be great in the UN environment.
Condoleezza Rice has praised
Bolton as a good negotiator
Republicans control the Foreign Relations Committee by 10-8, but most, if not all, panel Democrats are expected to oppose the nomination.
One of them, Senator Bill Nelson, a Florida Democrat, said Bolton had not been an effective arms negotiator and spoke to people in a condescending,inflammatory way.
''That's not the kind of representative of America that we want in the United Nations,'' Nelson said.
Committee Democrats also have circulated a portion of a two-year-old Senate Intelligence Committee report questioning whether Bolton had put pressure on a State Department intelligence analyst who tried to tone down language in a Bolton speech about Cuba's biological weapons capabilities.
On television talk shows on Sunday, committee Democrats Christopher Dodd of Connecticut, Joe Biden of Delaware and Jay Rockefeller of West Virginia cited the alleged pressure and other alleged incidents as among reasons they will oppose Bolton's nomination.
Committee Chairman Richard Lugar of Indiana hopes for a vote on Bolton's nomination on Thursday.
''That's not the kind of representative of America that we want in the United Nations''
Democrat Senator Bill Nelson
A tie could keep the panel from recommending Senate approval.
The outcome could depend on moderate Republican Senator Lincoln Chafee of Rhode Island. Chafee spokesman Stephen Hourahan said the senator was leaning towards supporting Bolton ''unless something surprising shows up'' at the hearing.
In preparations for the hearings, Democrats led by Biden have questioned Bolton's views on intelligence. They were granted access to four State Department officials and were permitted to examine some of its documents.
But Biden's spokesman, Norm Kurz, complained that the Democrats were not given everything they requested and were allowed only limited time for the interviews and only one day to look at the papers.
Carl Ford Jr, a former chief of the department's bureau of intelligence and research with whom Bolton apparently clashed, was scheduled to testify on Tuesday.
Since his nomination, Bolton has promised to work closely with other countries and members of Congress and said he had always supported "effective multilateral diplomacy".
Criticising the UN
As assistant secretary of state for international relations under former President George Bush Sr, Bolton helped organise the alliance that forced Iraq out of Kuwait.
Critics, though, recall his 1994 comment that it would not matter if the top 10 storeys of the 39-floor UN headquarters building in New York were lost.
Bolton's views on the UN have
not been charitable
He has said there is "no such thing as the United Nations" and asserted that the United States "is the only real authority the world has". He has also questioned whether the world body undertakes too many peacekeeping missions.
In February, he sharply criticised China for selling missile technology to Iran and other countries.
He has been critical of Europe's efforts to reach an agreement with Iran to curb that country's nuclear programme.
Targeting North Korea
During administration efforts two years ago to seek an agreement with North Korea over its nuclear programme, Bolton called that country's leader a ''tyrannical dictator''. Pyongyang officials refused to deal with him.
Bolton also helped lead US opposition to the International Criminal Court and the US' eventual withdrawal from the treaty creating the court.
His opponents have accused him of claiming without evidence that Syria and Cuba were trying to develop biological weapons.
Bolton would replace John Danforth, a former Republican senator from Missouri, who resigned after six months as UN ambassador.