John Bolton, the man who for years has sent chills through the ranks of diplomats at the United Nations, faced a tough confirmation hearing on Capitol Hill Monday.

Bolton has been nominated by President George W. Bush to be the new U.S. ambassador to the world body.

Bolton, currently the top arms control official in the U.S. State Department, is an administration hawk and has long been one of the UN's most vitriolic critics. In the past he's called it irrelevant and corrupt.

John Bolton said he would strengthen the UN, Capitol Hill, Monday. (AP Photo)

His confirmation by the U.S. Senate is in some doubt.

At least one Republican on the foreign relations committee is wavering and Democrats are wondering why Bolton would even want a job at an institution he seems to despise.

Over the years, Bolton's built a repertoire of anti-UN comments that Senate Democrats like John Kerry, Joe Biden and Barbara Boxer were delighted to play back.

"You have said there's no such thing as the United Nations," said Kerry. "You've said if the UN building in New York lost 10 storeys it wouldn't make a bit of difference."

"You've said that the peace enforcement operations and nation-building should, quote, be relegated to history's junk pile," said Biden.

Boxer accused him of having "nothing but disdain for the United Nations."

Bolton didn't deny or disavow his words. He said many of the statements were made as a private citizen, while others were quoted out of context.

Republican committee chair Richard Lugar reproached him for what he said was an abrasive, confrontational manner. "It should never be undertaken simply to score international debating points, to appeal to segments of the U.S. public opinion or to validate a personal point of view."

Democrats also accused Bolton, in his role as the senior arms control official in the State Department, of a penchant for distortion. There are allegations Bolton attempted to tweak intelligence information on weapons of mass destruction to suit the hawkish bent of the administration.

Senator Biden asked specifically if Bolton tried to have senior State Department weapons analysts reassigned when they refused to tell him what he wanted to hear.

Bolton confirmed the allegation. "I may have mentioned it to one or two other people, but then I shrugged my shoulders and I moved on," he said.

With a reputation as a hothead and a UN basher, Bolton's nomination to be U.S. ambassador has many at UN headquarters wondering if there's a hidden agenda. Many diplomats at the UN have quietly reacted to Bolton's nomination with disbelief.