WASHINGTON, June 7 — The sweeping immigration overhaul endorsed by President Bush crumbled in the Senate on Thursday night, leaving the future of one of the administration’s chief domestic priorities in serious doubt.

After a day of tension and fruitless maneuvering, senators rejected a Democratic call to move toward a final vote on the compromise legislation after Republicans complained they were not given sufficient opportunity to reshape the bill. Supporters of cutting off the debate got only 45 of the 60 votes they needed; 50 senators opposed the cutoff.

“We are finished with this for the time being,” said Senator Harry Reid, Democrat of Nevada and the majority leader, though he left the door open to returning to the issue later this year. “We all have to work, the president included, to find a way to get this bill passed.”

The outcome, which followed an outpouring of criticism of the measure from core Republican voters and from liberal Democrats as well, was a significant setback for the president. It came mainly at the hands of members of his own party after he championed the measure in the hope of claiming it as a major achievement on domestic policy in the last months of his administration.

It was also a disappointment for a bipartisan group of about a dozen senators who met privately for three months to broker a compromise that tried to balance a call for stricter border enforcement with a way for many of the 12 million people who are illegally in the country to qualify for citizenship eventually.

Senate conservatives fought the legislation from the start, saying that it rewarded those who broke the law by their illegal entry into the country. After winning a few important changes in the measure, Republican critics demanded more time and colleagues supported their calls for more opportunity to fight it out on the Senate floor.

Mr. Reid said the critics of the bill were simply stalling and would never be satisfied. He attributed the failure of the bill to Republican recalcitrance.

“We’ve done more than our share,” Mr. Reid said. “We’ve sent all the signals we can to get the president to help. It’s his bill.”

Mr. Reid did leave the door open to returning to the bill later this year.

The vote was the second attempt of the day to cut off a debate that had gone on for nearly two weeks, interrupted by the Memorial Day recess. On the initial showdown in the morning, the Senate fell 27 votes short of the 60 required; every Republican and 15 Democrats opposed the move.

“The majority is simply not going to get anywhere trying to stuff the minority,” said Senator Mitch McConnell of Kentucky, the Republican leader.

The morning vote sent Senate leaders and backers of the legislation scrambling, trying to reach an agreement to salvage the measure with the help of administration officials. Homeland Security Secretary Michael Chertoff was also consulted by phone.

As late as 6:30 p.m., Senator Edward M. Kennedy, Democrat of Massachusetts and a chief architect of the legislation, was asked if he had the votes to force an end to the debate. “It’s touch and go,” Mr. Kennedy said. “It’s extremely close at this time. Republicans have held their cards.”

If conservative Republicans continue objecting to the consideration of Republican amendments, Mr. Kennedy said, “we’ve really got a stalemate here.” But if the bill does not pass in the next few days, Mr. Kennedy said, the Senate will find another way to address the issue. “The issue is not going to go away,” Mr. Kennedy said. “We are not going to go away.”

The compromise legislation was announced on May 17 by authors who hailed it as a “grand bargain.” It held together through much of the debate because the negotiators — personified on the right by Senator Jon Kyl of Arizona, a Republican, and on the left by Mr. Kennedy — agreed to block proposals they thought would sink the measure. That led to such odd moments as when Mr. Kyl on Wednesday opposed an amendment he had helped write for last year’s unsuccessful immigration measure.

But the legislation began running into problems late Wednesday night and early Thursday morning as the Senate approved a Democratic proposal to limit a guest-worker program sought by business interests and backed by Republicans. Backers of the bill hoped to reverse that result if the measure moved forward.

“It is indispensable to have a guest-worker program to take care of the needs of the economy,” said Senator Arlen Specter, Republican of Pennsylvania, another of the central negotiators. “If we don’t, we will just encounter more people coming over illegally.”

At the same time, some Democrats were growing increasingly uneasy.

Senator Robert Menendez, Democrat of New Jersey, said the bill had become “more punitive and more onerous” because of amendments adopted in the last few days. He pointed, for example, to one that denies the earned-income tax credit to illegal immigrants who gain legal status under the bill.

Cecilia Muñoz, a vice president of the National Council of La Raza, the Hispanic rights group, said she had similar concerns. Changes approved by the Senate this week make the bill “not only more punitive, but also less workable,” Ms. Muñoz said.

Trying to bolster Democratic support, the Service Employees International Union urged senators Thursday to vote for a limit to the debate. In a letter to the Senate, Anna Burger, secretary-treasurer of the union, listed many serious objections to the bill, but said, “The time to move forward is now.”

The Leadership Conference on Civil Rights, a coalition of civil rights groups, also backed cloture, saying that “a small handful of immigration restrictionists’ in the Senate should not be allowed to prolong the debate indefinitely.”

Supporters of the bill said they would also try to change an amendment that gives law enforcement and intelligence agencies access to certain information in unsuccessful applications filed by illegal immigrants seeking legal status. This amendment, offered by Senator John Cornyn, Republican of Texas, was approved on Wednesday by a vote of 57 to 39.

Mr. Cornyn said his proposal would “remove the blinders from our law enforcement personnel, so they can investigate cases of fraud, wrongful conduct and other criminality” by illegal immigrants.

But Mr. Kennedy said immigrants would be extremely reluctant to apply for legal status if data from their applications could be shared with law enforcement agencies. “There are no individuals who are going to register for any of these programs — none — because all their information will be available,” Mr. Kennedy said.

Despite the strong Republican vote against ending debate, party leaders said throughout the day they wanted to reach some accommodation. Senator Trent Lott of Mississippi, the No. 2 Republican, urged his colleagues to stiffen their spines and try to resolve one of the nation’s most pressing problems. “Are we men and women or mice?

Are we going to slither away from this issue and hope for some epiphany to happen?” Mr. Lott asked. “No. Let’s legislate. Let’s vote.”

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Copyright 2007 The New York Times Company. Reprinted from The New York Times, National, of Friday, June 8, 2007.