WASHINGTON — Democrats forced the Senate on Thursday evening to postpone a vote on John Bolton as ambassador to the United Nations, demanding that the White House first hand over classified information about Bolton's conduct that it has refused for weeks to provide.

The move put off until at least June 7, when the Senate returns from its Memorial Day break, any decision on Bolton's embattled nomination, and it set Democrats and Republicans in the Senate at odds once again just three days after they reached a compromise intended to avert filibusters on judicial nominations.

Majority Leader Sen. Bill Frist, R-Tenn., described himself as "very, very disappointed" by what Sen. Harry Reid, of Nevada, the top Democrat, conceded was the "first filibuster of the year."

With Republicans holding a solid majority in the Senate, Bolton still appears poised to win confirmation if his nomination is put to an up or down contest. But a Republican-led effort to end debate on Bolton failed on a 56-42 vote, with the Republicans falling four votes short of the 60 necessary to bring Bolton to a vote.

Sen. Joseph R. Biden Jr., D-Del., who has led the fight against Bolton, said Democrats would agree to a vote on Bolton when the Senate returned from the recess. But Biden said Democrats would insist that the administration first provide information that the Senate has sought, related to a battle that Bolton waged in the summer of 2003 over intelligence assessments on Syria, and the names of Americans identified to Bolton by the National Security Agency as having been mentioned in intercepted communications.

The showdown on the Senate floor guaranteed more chapters to come in a battle over Bolton that has raged since he was nominated by President Bush more than 11 weeks ago. The struggle has been the sharpest in years over a nomination to a major foreign policy post, and it has marked by far the most fractious fight over an American ambassador to the United Nations since that institution was founded 60 years ago.

The White House expressed immediate dismay. "Just 72 hours after all the good will and bipartisanship, it's disappointing to see the Democratic leadership resort back to such a partisan approach," said Scott McClellan, the White House spokesman. Asked about the Democratic demands for additional documents, McClellan said: "They have all the information they need."

The vote at 6:15 p.m. on Thursday came at the end of 15 hours of speeches on the Senate floor that reflected bitter divisions between the parties. But with only one Republican, George R. Voinovich of Ohio, openly expressing opposition to Bolton, Biden ultimately cast the vote on whether to end debate not as a referendum about Bolton, but as a test of whether the Senate would "stand up for itself" in insisting on access to the information.

Three Democrats — Mark Pryor of Arkansas, Ben Nelson of Nebraska and Mary L. Landrieu of Louisiana — joined 53 Republicans in seeking to bring Bolton's nomination to a vote; 40 Democrats and one independent allied with them voted no on the motion to invoke cloture, as the Senate calls a bid to limit debate under its rules. Knowing that his side had lost, Frist then cast his vote with the minority, bringing their total to 42. The move was a tactical gesture intended to allow him to call for another vote later.

Voinovich voted with other Republicans in seeking to bring the matter to a vote, but also took the unusual step of returning to the Senate floor for a second straight day, to echo in emotional terms a message he had first delivered on Wednesday about the potential damage to the American image around the world if Bolton were sent to the United Nations.

"When was the last time so many individuals have emerged from an administration to send warning signs to the U.S. Congress about an individual?" Voinovich asked, in a reference to former colleagues of Bolton who have voiced strong public opposition to him. Voinovich added: "We owe it to the United States, our children and our grandchildren to heed this warning, and to ask our president, 'Please, Mr. President, find a better candidate to send to the United Nations.' "

Among Bolton's proponents, Sen. John McCain, R-Ariz., was among those who urged the Senate to cast aside the Democratic objections and vote on Thursday in favor of the nomination. "Elections have consequences," McCain said, urging the Senate to show deference to Bush's nominee.

Biden and Sen. Christopher Dodd, D-Conn., had spent most of the day insisting that their effort to prevent a vote on Bolton would not amount to a filibuster, on the grounds that they would be willing to back down from their position if the administration provided the documents. But after the vote, Frist mocked that position, suggesting that the success of 42 senators in preventing a vote on Bolton "looks like a filibuster, sounds like a filibuster" and even "quacks like a filibuster."

Reid did not quarrel with that assessment but said he hoped the Democrats' successful effort to block the vote on Bolton would be both "the first filibuster of the year, and maybe the last." Still, Reid said in addressing his Republican counterpart: "How can we work together when information is not supplied?"