Senate Foreign Relations Committee chairman Richard Lugar, an Indiana Republican, opened a second day of debate on the nomination with a strong endorsement of Mr. Bolton, currently the undersecretary of state for arms control and international security.
"I believe he will be an outstanding representative of our country, and an able diplomat at the United Nations," senator Lugar said.
But Democrats are concerned by allegations he mistreated subordinates and sought to shape intelligence to meet ideological ends.
Senator Barbara Boxer, a California Democrat and another member of the Foreign Relations Committee, had blocked the nomination from coming to the Senate floor for debate until earlier this week:
"Mr. Bolton, he threatened retribution when the intelligence does not conform to his views," she said. "It is a disaster to promote someone like that."
Democrats have signaled they may try to delay the vote on the nominee until next month, unless the administration turns over classified documents they say could shed more light on whether Mr. Bolton tried to tamper with intelligence assessments.
At least one Republican shares Democrats' concerns. Senator George Voinovich of Ohio has sent a letter to his Senate colleagues saying confirming Mr. Bolton would send the wrong message to the world community.