Both sides said that without a quick resolution of the differences they would not have a vote on the final legislation before Congress leaves for its spring recess on Friday, raising the possibility that the painstakingly negotiated compromise might unravel as it is exposed to intense political scrutiny during the two-week Congressional break.

The plan would create a temporary worker program that would allow 325,000 foreigners to fill jobs in the United States each year. And, if passed, it would mark the most sweeping immigration accord in two decades.

The late-night battle burst into the open many hours after Senate leaders had resolved their substantive differences over the thorny question of legalization. Conservatives, who condemned the compromise as little more than amnesty for lawbreakers, sought to offer amendments to the bill. But Democrats refused to allow votes on the amendments, saying they were intended to delay the process and gut the legislation.

And as members of the two parties took to the Senate floor to accuse each other of trying to derail the legislation, negotiators warned that the failure to resolve the procedural disagreements was jeopardizing what everyone described as a substantive agreement.

"The fact that we did not act tonight is a huge blow," said Senator John McCain, Republican of Arizona, and a main architect of the deal.

Under the Senate agreement, illegal immigrants who have lived in the United States for five years or more, about seven million people, would eventually be granted citizenship if they remained employed, had background checks, paid fines and back taxes and learned English.

Illegal immigrants who have lived here for two to five years, about three million people, would have to travel to a United States border crossing and apply for a temporary work visa. They would be eligible for permanent residency and citizenship over time, but they would have to wait several years longer for it.

Illegal immigrants who have been here less than two years, about one million people, would be required to leave the country altogether. They could apply for spots in the temporary worker program, but they would not be guaranteed positions.

President Bush praised the Senate's efforts and urged lawmakers "to work hard and get the bill done" before the recess.

And Republican and Democratic leaders, who had battled so bitterly on Wednesday that an agreement seemed in jeopardy, stood side by side on Thursday morning, hailing the deal as a historic decision that would enhance national security by bringing illegal immigrants out of the shadows while meeting the nation's needs for labor.

Flanked by more than a dozen Republican and Democratic lawmakers, Senator Bill Frist of Tennessee, the majority leader, stood before a crush of television cameras on Thursday morning and called the compromise "a huge breakthrough."

But as the day wore on, tensions began to rise as Republicans insisted that the Democrats allow a vote on several of their amendments.

One amendment would require the Department of Homeland Security to certify that the border was secure before creating a guest worker program or granting legal status to illegal immigrants. Another would have the legalization program bar illegal immigrants who had deportation orders or had been convicted of a felony or three misdemeanors. Democratic critics of the proposals said they were intended to ensure that the legalization process would never be implemented.

Republican supporters of the compromise, including Mr. McCain, said he and others would easily vote down such amendments if they were brought to a vote.

Meanwhile, Democrats said they were still awaiting detailed assurances from Mr. Frist and others that Republicans would defend the agreement in the face of strong conservative opposition when House and Senate negotiators sit down to reconcile their bills.

Any immigration bill that passes the Senate must first be reconciled with the tough border security bill that passed the House in December. And House conservatives warned on Thursday that they would reject the Senate compromise. Republicans and Democrats alike agreed that the compromise measure would pass easily if lawmakers were allowed to vote for it. But parliamentary procedures allow even a small number of senators to prevent a vote from occurring before the spring recess.

"Here we are now and we have made absolutely no progress," said Mr. Frist, taking to the Senate floor.

Senator Harry Reid of Nevada, the Democratic leader, countered that Republicans seemed intent on bogging down the process with countless amendments. "Republicans seem intent on delaying and defeating this compromise," Mr. Reid said.

In recent weeks, both parties have been under pressure to pass legislation that would resolve the fate of the 11 million or so illegal immigrants in the United States. President Bush has raised the issue, business groups have lobbied fiercely for it, and tens of thousands of immigrants and their supporters nationwide have poured into the streets, reflecting the growing political muscle of Hispanics.

But the debate has deeply divided the Republican Party, and conservatives warned on Thursday that the Senate compromise sounded too similar to the amnesty enacted by President Ronald Reagan in 1986, which granted legal status to nearly three million illegal immigrants.

"This compromise would repeat the mistakes of the past, but on a much larger scale because 12 million illegal immigrants would still be placed on an easier path to citizenship," said Senator John Cornyn, Republican of Texas.