The fate of the immigration bill comes down to Senate Republicans' leader, Mitch McConnell, and whether he sides with President Bush and Democratic leaders or with rank-and-file members of his own party.
His choices: Accept Democrats' demands to finish the bill this week without major changes, or rally with his Republicans who say the bill is bad and getting worse, and want more time to fight the proposal.
"The only thing that can save us now is if McConnell made an appeal to Republicans that this thing is out of control and we need to step back from it, and we're not getting the amendments we want," said Sen. Jim DeMint, South Carolina Republican. "He could do that, but I'm not sure where he is right now."
Debate on the bill resumes today, and Republican leaders will have to decide quickly whether to push for a drawn-out debate.
Mr. McConnell, Kentucky Republican, has been on both sides of the immigration issue. He voted for last year's bill and has said this year's version is even better, but he also voted last week to strip out the central plank of the bargain: the legalization program for illegal aliens.
Republicans use words like "confusion" to describe their party's approach, and said that has hurt their efforts to change or defeat the bill. They also say Mr. McConnell and Mr. Bush have given up the best bargaining chip by making it clear they have to have a bill.
"The president has said he's going to sign whatever we send him, so we started with our legs cut out from under us," Mr. DeMint said. "You shouldn't be negotiating if you're not willing to walk away from the table."
Democrats have been far better organized. Majority Leader Harry Reid, Nevada Democrat, has blocked Republican amendments and given Democrats time to counter them.
The bill, negotiated behind closed doors by a bipartisan group of senators and two Bush Cabinet secretaries, offers immediate legal status and a path to citizenship to illegal aliens, creates a temporary-worker program for hundreds of thousands of future foreign workers, and rewrites the definitions that govern how immigrants are selected.
Republicans said their leaders have been good at preserving members' rights, but that this next week will be a critical test.
"I think we're going to have to have some very serious discussions in our policy committee luncheon on Tuesday about the way forward," said Sen. John Cornyn, Texas Republican, who has about two dozen amendments he would like to offer. "I, for one, am going to object strongly to jamming this bill through by the end of the week."
He and other Republicans say there is no way a bill can be completed this week and they will demand more time. That will force Mr. Reid to make a procedural move to end debate, which will require 60 votes.
Some Republicans have failed to take advantage of their best chances to defeat the bill. Just nine of them voted for a Democrat-sponsored amendment to end the guest-worker program after five years -- an amendment that failed by just one vote, and could have killed the entire bill if it had passed.
More than 100 amendments have been filed, but just seven have received roll-call votes. Last year, 32 amendments had roll-call votes.
Spokesmen for both Mr. McConnell and Mr. Reid said the final decisions haven't been made on which amendments will receive votes, and the two men will meet today or tomorrow to hash that out.
Jim Manley, Mr. Reid's spokesman, said the Democratic leader insists the bill be finished by Friday.
"It's a thin line between allowing them the right to offer their amendments and the need to finish this up next week," Mr. Manley said.
Democrats will insist on votes on their amendments to broaden the group of family members that can be sponsored for immigration and to tighten further the guest-worker program.
Meanwhile, Republicans likely will insist on amendments to broaden the list of those ineligible for legalization to include felony drunken drivers, gang members and those who have been ordered deported; deny benefits like the Earned Income Tax Credit to newly legalized aliens; make legalized aliens compete for green cards with would-be legal immigrants; and advance to an earlier date the "touchback" provision requiring illegal aliens to leave the country briefly.
Democrats will counter some of those Republican amendments with what are called "side-by-side" votes -- amendments that appear to address the same problem but don't go as far as Republicans' solutions would. They also may pair Democratic amendments with Republican amendments in an effort to get more senators to sign off on the compromise.
"The smart thing would be getting buy-in by significant chunks from both sides," said Tamar Jacoby, senior fellow at the Manhattan Institute, who is pressing for the bill. "If you could do a few big things for both sides and get buy-in from significant chunks, then it becomes about mopping up."
Without Mr. McConnell, Republicans say, it is unlikely they can block the bill, and they doubt they can find enough support among Democrats to help them.
The Democrats divide into two groups: those such as Sen. Byron L. Dorgan of North Dakota, who oppose the temporary-worker program as bad for American workers, and those such as Sen. Robert Menendez of New Jersey, who say the bill is unworkable because it is too harsh on aliens.
Mr. Dorgan's spokesman, Barry Piatt, said his boss will vote against the bill if it contains the guest-worker program but said he does not know whether Mr. Dorgan would be willing to join a filibuster.
Republicans doubted many Democrats will oppose the bill in the end.
"I think the opposition from the left is more posturing and is less likely to provide votes if it is a close question," said Sen. Jeff Sessions, Alabama Republican. "I think that opposition is going to evaporate."