Republicans fell four votes short of the 60 votes needed to end debate on the nomination, putting off a final vote on Bolton's confirmation until after the Memorial Day recess.
Bolton, who is undersecretary of state for arms control, faced opposition among mostly Democratic senators and a handful of Republicans, notably Sen. George Voinovich of Ohio, who urged senators Wednesday not to vote for the nominee just because he was President Bush's choice.
Rather, Voinovich said, senators should consider Bolton's record of abusive and erratic behavior as reasons to disqualify him from the post.
Voinovich expressed concern some U.N. members "will use Mr. Bolton as part of their agenda to further question the integrity and credibility of the United States of America and to reinforce their negative U.S. propaganda," Reuters reported.
Other Republicans defended the choice of Bolton, including Senate Foreign Relations Committee Chairman Richard Lugar of Indiana.
"Opponents have argued that Secretary Bolton's personality will prevent him from being effective at the U.N., but his diplomatic successes over the last four years belie that expectation," Lugar said.
Democrats argued there was evidence that Bolton tried to exaggerate intelligence assessments of the weapons capacity of several countries, bullied subordinates and was temperamentally unsuited to the sensitive diplomatic post. They said no vote should be held until the Bush administration releases classified and other information on Bolton that they requested weeks ago.
"That this individual who engaged in such reprehensible behavior ... should be given the position of U.N. ambassador, to represent the United States at this critical hour, I think is a massive mistake," said Sen. Christopher Dodd, D-Conn., on Thursday, according to Reuters.
But Sen. Mel Martinez, R-Fla., said the United States would benefit from having Bolton's representation in the United Nations. Martinez complained that Bolton's record "has been trashed repeatedly, oftentimes with scant little evidence," and said it is "in our national interest sometimes to have direct blunt-speaking people."
The procedural move came two days after centrist senators from both parties forged a compromise over use of the filibuster in blocking President Bush's judicial nominees.
Senate Majority Leader Bill Frist, R-Tenn., denounced the Democrats' decision to put off a final vote, saying it amounted to a continued effort at obstruction.
"John Bolton, the very first issue we turned to, we got what looks to me like a filibuster," Frist said. "It certainly sounds like a filibuster ... it quacks like a filibuster."
Sen. Joseph Biden, D-Del., rejected the accusation, saying the Democrats simply wanted to see more information on the nominee before moving to a final up-or-down vote.
"We are willing to vote 10 minutes after we get back in session, if in fact they provide the information," Biden said.