For the second time in two months, Democrats used a procedural vote to block Mr. Bolton's nomination because the Bush administration has not turned over information they requested about Mr. Bolton's use of intelligence material.
The Senate voted 54 to 38 to cut off debate on the nomination, falling short of the 60 votes needed in the 100-seat chamber to move to an up-or-down confirmation vote.
One Republican, Senator George of Ohio, a member of the Foreign Relations Committee who has voiced his concerns about Mr. Bolton's fitness for the job, voted with the Democrats.
Critics say Mr. Bolton, whose most recent position was as undersecretary of state for arms control and international security, may have improperly used intelligence to intimidate officials who disagreed with his views.
Democrats are seeking classified National Security Agency intercepts that Mr. Bolton had sought to see if they contain the names of those whose intelligence assessments the nominee had disputed. They also are requesting internal communications leading up to testimony Mr. Bolton gave Congress on Syria's weapons' capabilities.
Democrats are angry the administration has not handed over the information.
"I am aware, as we all are, on both sides of the aisle, of the sometimes admirable, but most times, excessive obsession with secrecy on the part of this administration," said Senator Joe Biden of Delaware, the top Democrat on the Senate Foreign Relations Committee. "But not withstanding that, Mr. President, we should not forfeit our responsibility to accommodate that obsession."
In refusing the Democrats' request, the White House says it already has been very forthcoming with information on Mr. Bolton.
Republicans dismiss Democrats' concerns as unsubstantiated or irrelevant, and say their request for more information is nothing more than a delaying tactic.
"This is the definition of a fishing expedition, which the sole goal is to bring down a nominee because of differing policy views," said Senator George Allen of Virginia.
Republicans note that Mr. Bolton would be easily confirmed by the Senate, which their party controls with a 55-seat majority, if the Democrats would allow the vote to take place.
Just hours before the vote, President Bush praised Mr. Bolton as a well-qualified candidate who will approach the task of reforming the United Nations seriously.
"I nominated John Bolton to be the Ambassador to the United Nations for a reason," the president said. "The American people know why I nominated him: because the U.N. needs reform.
Mr. Bush did not directly respond to questions about whether he would name John Bolton as a recess appointment. The president could install Mr. Bolton at the United Nations while Congress is away for the July 4th Independence Day holiday recess or during their month-long August vacation. But under the recess appointment rules, Mr. Bolton would be able to serve only until January 2007.