The legislation faces a make-or-break vote this morning when senators will decide whether to cut off debate and move to a final vote tomorrow. If it does not get the 60 votes necessary, Majority Leader Harry M. Reid (D-Nev.) has said he will pull the bill, all but dashing hopes for any meaningful legislation this year.

Top legislative aides in both parties predicted today's vote would be very close but would fall short of keeping the proposal alive.

Sen. Jim DeMint (R-S.C.), a key opponent, crowed last night that "they tried to railroad this through today, but we derailed the train." Another opponent, Sen. Jeff Sessions (R-Ala.), said, "I would say to my colleagues: Let's end this thing."

Key Democrats who were on the fence also raised questions. Sen. Robert Menendez (D-N.J.) said the failure of his amendment to bolster family reunification visas "makes it more difficult to vote in favor" of ending debate. The reunification provision was voted down 55 to 40.

Last night's stall came after a day that had left the bill's proponents optimistic. The defeat of provisions intended to toughen the bill or soften its restrictions suggested that the core of the "grand bargain" was holding in the Senate's second attempt to pass an immigration bill supported by the White House.

One key amendment rejected yesterday was a Republican proposal to require all adult illegal immigrants to return to their countries temporarily to qualify for a special new visa.

The provision, an amendment offered by Sen. Kay Bailey Hutchison (R-Tex.), was defeated, 53 to 45. But a similar amendment that would require only heads of households to return to home countries is expected to fare better if it comes to the floor, after the vote to shut off debate.

The defeated amendments were among at least 26 measures up for consideration. Some are designed to stiffen the bill in response to criticism from conservatives, while others are aimed at weakening provisions that immigrants' rights advocates or employers consider too burdensome.

The overall bill would create a path to U.S. citizenship for the nation's 12 million illegal immigrants and establish a new guest-worker program that would allow hundreds of thousands more to enter the country to take jobs that President Bush says Americans do not want. It also would pour billions of dollars into an effort to tighten border security.

A number of Republican senators have bucked the White House on the bill, vowing to kill it because its legalization provisions amount to "amnesty" for illegal immigrants, a deeply unpopular concept among many conservatives. The bill also has come under attack from liberals and immigrants' advocates, who oppose some of its restrictions and its emphasis on awarding visas based more on skills than on the family connections that are given primacy under the current system.

One of the defeated amendments, offered by Sen. Christopher S. Bond (R-Mo.), would have denied citizenship to all illegal immigrants. It lost, 56 to 41. Another amendment introduced by Sen. James Webb (D-Va.), which would have given legal status to only those illegal immigrants who have lived in the United States for at least four years, went down 79 to 18. The bill would extend eligibility for legalization to those who arrived by Jan. 1, 2007.

An attempt to liberalize the bill also failed, as senators voted 56 to 41 against an amendment by Sen. Christopher J. Dodd (D-Conn.) that would have increased the availability of green cards for relatives of those already here. Republicans strongly opposed the amendment because it would have violated a key tenet of the "grand bargain" behind the bill: shifting the allocation of green cards to the skills-based system favored by employers.

Hutchison's amendment would have required all adult illegal immigrants eligible to work to return to their home countries within two years after receiving a temporary permit to remain in the United States. It would have applied to those seeking a new "Z Visa," a category designed to give probationary status to illegal immigrants before they become eligible to apply for green cards, which denote permanent legal resident status.