The fragile immigration reform bill crafted in the Senate faces sudden death Thursday when a second vote is scheduled to shut down a filibuster.

If supporters don't get the 60 votes they need to end debate, then the bill is dead since the Senate schedule does not allow any more time to revive the legislation.

In an unusual admission, two leadership aides — one Democrat, one Republican — told FOX News that they have no idea how the vote will go. Vote-counting is something that is normally down to a science in the Senate chamber. The vote to end debate, called "cloture," is expected at 10:30 a.m. ET.

The mood in the Senate took a perceptible shift Wednesday as lawmakers decided not to "table," or send into permanent limbo, one of the 27 amendments approved for debate. That amendment was designed to weaken worker verification standards that are key to the measure's approach to reducing the ability of illegals to obtain work with fraudulent documents.

The Senate vote, 45-52, not to kill the amendment stops all momentum toward passage, something the White House and bipartisan Senate leadership hoped to achieve by Friday.

Under a complicated and rarely-used procedure, the immigration bill could only move forward if amendments offered to it were defeated through tabling. But after the tabling vote was defeated, Senate Majority Leader Harry Reid could be heard on microphones saying that he and those who support the bill were now "stuck."

Backers of the bill allowed votes on amendments they felt confident they could defeat, thereby keeping intact a carefully balanced, bipartisan compromise that has as its core policy goals tightening border security, creating a guest worker program and establishing legal protections for an estimated 12 million illegal immigrants here now.

But the Senate refused to defeat an amendment offered by Sens. Charles Grassley, R-Iowa, Barack Obama, D-Ill., and Max Baucus, D-Mont. It was designed to lift some of the bill's requirements on employers to verify records that illegals present to obtain work. Grassley's said the amendment was designed to reduce what he called excessive compliance costs on businesses. Immigration groups also supported the change for reporting rules they described as too cumbersome.

The Senate will conduct no more votes on nearly 20 more amendments to the Senate bill until it conducts the next procedural vote early Thursday. The vote, on whether to shut down debate and move to final passage, now appears likely to lose the extra bit of Republican support it had picked up during Tuesday's vote to continue working on the comprehensive package.

Tuesday's vote to move onto the bill was 64-35, four votes more than was needed to keep the legislation alive. But if five Republicans are lost as a result of the amendment, then it's bye-bye bill.

Late Wednesday, two Republicans, Sens. Kit Bond of Missouri and Richard Burr of North Carolina, told FOX News that they will switch their vote and not try to prevent the filibuster. One Democrat who never votes against cloture, Sen. Ben Nelson of Nebraska, also told FOX News that he will vote "no" this time.

"Enough is enough. There have been seven cloture votes I've voted yes on. This is an extraordinary circumstance. But enough is enough. ... This clay pigeon is looking like a dead duck," Nelson said.

The clay pigeon refers to the amendments that were introduced as one large package shot into 27 smaller pieces to prevent anyone from being able to block the bill. Critics have said Reid's decision to use the clay pigeon was high-handed, and both Bond and Burr said they don't consider it to be a fair process.

At least nine other senators told FOX News they are undecided or leaning toward a no vote. They include Democratic Sens. Mark Pryor, Jim Webb, Jeff Bingaman, Robert Menendez and Republican Sens. Ted Stevens, Lisa Murkowski, Pete Domenici, Judd Gregg and John Ensign.

A senior Democratic leadership aide involved in the crafting of the compromise bill said Reid and Minority Leader Mitch McConnell are meeting to talk about where they can get votes. Democrats have said they are at their ceiling and it would be hard to convince others like Sens. Evan Bayh, Mary Landrieu, Claire McCaskill and Debbie Stabenow to change their votes and support cloture.

Domenici, a one-time ardent supporter of the bill who is up for re-election in 2008, told FOX News that Republicans are "getting hammered here at home and for what? Something that doesn't even have the chance of becoming law? No. This bill is going down. ... Why are we having all these big amendments? They (supporters) don't even know what's in this bill. We learned it's not even enforceable. I just don't think I can support this bill."

Domenici added that a vote Tuesday night in the House GOP conference was very telling for him. Only 23 Republicans of 201 voted against a resolution of disapproval of the Senate measure.

Five Amendments Sit Indefinitely on the Table

Up until the late moving events, none of the votes in Wednesday's debate altered the underlying bill.

The Senate tabled a Republican proposal by Bond to deny green cards to unlawful immigrants. The vote was 53-45 to table an amendment by Sen. Kay Bailey Hutchison, R-Texas, that was one of several proposals designed to respond to conservatives who decry President Bush's immigration bill as a form of amnesty.

The bill could grant lawful status to as many as 12 million illegal immigrants as long as they passed background checks and paid fines and fees. Heads of household seeking permanent legal residency would have to return home to apply for green cards, however.

Without her amendment, Hutchison said shortly before the vote, "the amnesty tag that has been put on this bill will remain. It is the key issue in the bill for the American people."

In addition, the Senate killed, by a 56-41 vote, an amendment by Sen. Christopher Dodd, D-Conn., to provide more green cards for parents of U.S. citizens. By a 55-40 margin, it tabled a proposal by Sen. Robert Menendez, D-N.J., to give family members of citizens and legal permanent residents more credit toward green cards in a new merit-based points system.

An amendment by Sen. Jim Webb, D-Va., also defeated, would have restricted legal status applications to those who have been in the United States for four years. The bill would allow anyone in the U.S. as of Jan. 1, 2007, to be eligible.

Webb said his proposal would raise the public's comfort-level with granting lawful status to illegal immigrants.

"People in this country who traditionally would be supporting fair immigration policies, but who are worried about the legalization process in this bill, would come forward and support this," Webb said.

His amendment would scrap the return-home requirement, which he called unrealistic and impractical.

The Associated Press contributed to this report.