The chairman of the Senate panel, Senator Richard Lugar, an Indiana Republican, does not believe that more allegations against Mr. Bolton would change any minds in the committee.
The Washington Post newspaper Monday reported that Mr. Bolton withheld sensitive information on Iran from his bosses, Secretary of State Condoleeza Rice and her predecessor, former Secretary Colin Powell. The newspaper quotes State Department sources citing at least a dozen instances when Mr. Bolton refused to forward information during his four years as undersecretary of state for arms control and international security.
Last week, a former State Department official, Carl Ford, told the committee Mr. Bolton sought the removal of an analyst who disagreed with Mr. Bolton's assessment that Cuba had banned weapons.
Democrats are troubled by the allegations, which Mr. Bolton has denied. They are also concerned about his hardline views on North Korea and Iran, as well as his past criticisms of the United Nations, which he has called corrupt and irrelevant.
Democrats are expected to oppose the nomination. Senator Joe Biden of Delaware is the top Democrat on the committee.
"I think that John Bolton's lack of credibility on intelligence matters, John Bolton's strong and in some areas very respected ideological view of foreign policy intelligence are admirable, but they're not admirable for someone running a Cabinet-level position, one of the largest embassies in the world, which is the, essentially, the embassy of the United States at the U.N," he said on Fox News Sunday.
The Foreign Relations Committee was to have voted on the nomination last week, but Democrats asked for more time to gather information from opponents of Mr. Bolton.
Democrats hope to persuade just one Republican on the committee to oppose Mr. Bolton, which could force a tie vote that would block the nomination from reaching the full Senate.
Senator Chuck Hagel, a Nebraska Republican, said on CNN's Late Edition Sunday that he is inclined to support the nominee, although he said that could change.
"I have been troubled with more and more allegations, revelations, coming about his style, his method of operation," he said. "We need a uniter. We need a builder. We need someone who will reach out to our friends and our allies at the United Nations."
Chairman Lugar told Fox News Sunday that the overriding issue for senators to consider is that President Bush has the trust of Mr. Bolton to reform the world body:
"The president of the United States has nominated John Bolton because he believes there should be very substantial United Nations reform," he said. "He thinks Bolton is an agent of reform, has really the skills and the savvy and what have you, and maybe the combative nature, to bring about change."
White House spokesman Scott McClellan told reporters Monday that Mr. Bolton "is exactly the kind of person that we need at the United Nations." He is an effective diplomat who has a proven record of results.