WASHINGTON (CNN) -- After hours of heated exchanges, the Senate failed Thursday evening to close off debate on President Bush's nomination of John R. Bolton to be ambassador to the United Nations.
The cloture vote would have paved the way for a vote on whether to confirm the controversial nominee.
Republicans needed 60 votes to cut off the debate; they fell short by four, with a 56-42 breakdown.
The debate on Bolton's nomination will now likely resume next month after senators return from their weeklong holiday break.
Senate Majority Leader Bill Frist had said he wanted a vote on the nomination before the weeklong Memorial Day holiday.
"It does disappoint me," Frist said after the vote. "It looks like we have, once again, another filibuster."
Senate Minority Leader Harry Reid took issue with Frist's comment.
"This is the first filibuster of the year, and maybe the last. (I) hope so," the Nevada Democrat said.
After cloture, Bolton would need a simple majority to be confirmed by the Senate, which has 55 Republicans, 44 Democrats and one independent.
Two key Democratic senators -- Christopher Dodd of Connecticut and Joe Biden of Delaware -- asked their Democratic colleagues Thursday to vote against closing the debate.
The two said they wanted more time for the Bush administration to turn over requested documents relating to Syria and to 10 instances of communications intercepted by the National Security Agency.
The senators said Bolton had once sought those intercepts as the undersecretary of state for arms control.
In a letter to their peers, the two Democrats said the administration's refusal to provide the documents "is a threat to the Senate's constitutional power to advise and consent."
"The only way to protect that power is to continue to demand that the information be provided to the Senate," the senators said. "The only means of forcing the administration to cooperate is to prevent a final vote on the nomination today."
On the Senate floor, the debate was passionate on both sides of the aisle.
Sen. Barbara Boxer, a California Democrat, pointed to negative comments Bolton has made about the United Nations.
Bolton said during a Federalist Society forum in 1994: "If the U.N. secretary building in New York lost 10 stories, it wouldn't make a bit of difference."
Boxer asked: "What kind of credibility does he have walking onto the floor of the United Nations?"
Sen. Ted Kennedy, a Massachusetts Democrat, added: "The United Nations is the one and only organization where the nations of the world can link their unique strengths in a realistic hope of building a peaceful future for all humanity.
"We need a representative at the United Nations who supports that vision and is committed to that future for us all," he said. "John Bolton is not that person for that job."
But Sen. John McCain, an Arizona Republican, said Bolton would represent the nation well.
"The United Nations needs the presence of a tough, hard, dedicated individual that has been already, already confirmed in various posts four times by this body," he said.
Sen. Judd Gregg, a New Hampshire Republican, called for Democrats to allow Bolton's nomination to come to a vote.
"I hope we will move forward to a final vote on Mr. Bolton this afternoon and people who think he is the wrong choice and may have policy differences with the president on how we are pursuing liberty and democracy across the globe can vote against him on that basis," Gregg said.
At least one Republican, Sen. George Voinovich of Ohio, has said he will not vote for Bolton because he believes the State Department official could undermine Bush's foreign policy.
But the offices of other Republican moderates -- Lincoln Chafee of Rhode Island, Lisa Murkowski of Alaska, Susan Collins of Maine and Chuck Hagel of Nebraska -- indicated they would support Bush's pick.
Democrats have charged Bolton lacks the diplomatic skills for the job.
At his confirmation hearings last month, senators heard testimony that Bolton bullied subordinates and tried to get intelligence analysts who disagreed with him fired or reassigned.
One of Bolton's former State Department colleagues testified last month that Bolton was "a quintessential kiss-up, kick-down sort of guy."
He also was accused of manipulating intelligence to fit his preconceived notions and misleading the committee.
Bolton's supporters, including the White House, argue he is a seasoned government official who can spearhead reform at the United Nations.
The Republican-dominated Senate Foreign Relations Committee voted 10 to 8 along party lines two weeks ago to send the Bolton nomination to the full Senate. But the committee withheld its endorsement, something rarely done.