The Senate late Thursday voted 56 to 42 to cut off debate on Mr. Bolton's nomination.
Republican leaders could not get the required 60 votes necessary in the 100-seat chamber to end debate and move to a confirmation vote.
"It does disappoint me," said Majority Leader Bill Frist.
The action comes after a contentious political battle over Mr. Bolton's nomination.
Democrats are united in their opposition to Mr. Bolton, whom they say is unfit to represent the United States at the world body.
They expressed concerns about allegations that Mr. Bolton, currently undersecretary of state for arms control and international security, sought to shape intelligence to meet ideological ends.
Senator Chris Dodd of Connecticut said those allegations are particularly troubling in light of the faulty intelligence the United States used in making the case for war in Iraq. "The issue goes far beyond the individualities at stake here. It goes to the heart of whether or not we are going to have credible intelligence upon which we as members of this Congress can believe and our allies around the world and those we seek to find support on various foreign policy matters will understand the purposes to which we are seeking their support. That is what I worry about more than anything else," he said.
Senator Dodd and other Democrats are seeking classified documents from the administration they say could shed more light on whether Mr. Bolton tried to tamper with intelligence assessments.
Republican leaders say investigations into Mr. Bolton nomination have been exhaustive, and that the allegations against the nominee are overstated or unsubstantiated.
Republicans say Mr. Bolton is a determined reformer who is needed at the United Nations, especially in the wake of the UN oil-for-food scandal. "A vote for John Bolton is a vote for U.N. reform," said Senator Frist.
But Senator Russ Feingold, a Wisconsin Democrat, questions whether Mr. Bolton, an outspoken critic of the United Nations, is qualified to reform the world body. "Mr. Bolton's record suggests his personal animosity toward the United Nations is so great that he cannot effectively lead the charge of reform that can make this vital but deeply-flawed institution stronger and more effective," he said.
One Republican agrees with the Democrats' concerns. Senator George Voinovich of Ohio says Mr. Bolton's brusque manner could hamper U.S. efforts to mend ties with allies frayed after the Iraq war and other matters:
"I would like to point out that Mr. Bolton will be going to the United Nations to do more than just push forward U.N. reforms and sharp elbows. He is there to be the U.S. representative to the world. Do we really want the supreme quality of our next U.N. representative to the world to be sharp elbows? Don't we need a man with superior interpersonal skills who can bring people together, form coalitions and inspire other countries to agree with his point of view?," he said.
Thursday's Senate action comes just days after a bipartisan agreement averted a confrontation on President Bush's judicial nominees. Democrats agreed to allow several controversial nominees to get up-or-down votes while promising not to block other nominations except under extraordinary circumstances. Republicans agreed not to ban Democrats' ability to block the nominees.
Ahead of Thursday's vote, Senator John McCain, an Arizona Republican who is credited with brokering the deal on the judicial nominees, unsuccessfully appealed to Democrats not to block Mr. Bolton's nomination. "I'm not asking my colleagues who disagree and do not want Mr. Bolton there. I respect their views. But let's go ahead and give him an up or down vote before we go into recess for a week," he said.
Majority Leader Frist says the Senate will reconsider the nomination when it returns from a week-long recess in early June.