Less than a week after touting a breakthrough on a new immigration reform bill, lawmakers from both sides of the aisle on Tuesday started tearing apart the original proposal on how to handle the 12 million illegal immigrants living in the United States.

The bill's future is unclear after the Senate test vote late Monday garnered the 60 votes needed for lawmakers to begin debating the 1,000-page document but Senate leaders agreed to postpone finishing the legislation until next month.

Opposition to the bill, which many senators complain has only just made it in final form into their inboxes, is coming from the right and left. Normal allies of comprehensive immigration reform, pro-immigrant groups and many business consortiums are looking askance at the legislation. Not one union group has voiced support for it.

Some Republicans call the bill amnesty with a renewable visa system while some Democrats oppose the proposal that makes skills and education more important than family ties.

The Senate on Tuesday will take up a handful of amendments to the bill from Republicans and Democrats.

Some of those amendments are being offered by:

— Sen. Jeff Bingaman, D-N.M., who wants to cut the temporary worker program in half to 200,000 per year;

— Sen. James Inhofe, R-Okla., who wants to make English the official language of the United States and prevent entry if an immigrant can't pass the current English proficiency test;

— Sens. Barbara Boxer, D-Calif., and Byron Dorgan, D-N.D., who propose killing the guest worker program;

— Sen. John Cornyn, R-Texas, who wants to allow federal law enforcement to use information from immigrant visa applications in investigations;

— Sens. Patrick Leahy, D-Vt., and Robert Menendez, D-N.J., who want to give more weight to families for earning green cards; and

— Sen. Jeff Sessions, R-Ala., who plans to speak against the bill for a few hours Tuesday morning with votes on amendments expected in the afternoon.

In a nod to that opposition, Senate Majority Leader Harry Reid conceded that the Senate won't be able to complete the bill before a hoped-for Memorial Day deadline.

"It would be to the best interests of the Senate ... that we not try to finish this bill this week. I think we could, but I'm afraid the conclusion wouldn't be anything that anyone wanted," Reid, D-Nev., said.

Negotiators who hammered out the deal behind closed doors after months of meetings with Bush administration officials said they will meet each morning the Senate is in session during the debate to review the slate of amendments for the day and decide a strategy for supporting or defeating those amendments.

The negotiating group has about a dozen members and could easily be thwarted if liberal Democratic critics join opposing Republicans to pass any given measure. A number of the original negotiators have already left the group in opposition, including Cornyn, Leahy and Menendez. Still others in the group are withholding their support for now, like Georgia Republican Sens. Saxby Chambliss and Johnny Isakson.

The bill beefs up border security and employer verification, sets up a temporary worker program, legalizes the vast majority of the approximately 12 million illegals currently in the United States and sets up a merit-focused point system that still heavily weighs family connections for earning a green card.

Currently, about two-thirds of legal permanent residency cards are family-based, while one third are employment-based. Legislative experts agree that current ratio would not substantially change, though family migration is drastically curtailed. Only minor children and spouses can accompany green card holders, with a limited number of visas granted to "grandparents." Others would have to apply for their own green card.

Illegals, once they come out of the shadows and register, receive a temporary card and eventually a special "Z visa" to work indefinitely in the U.S, a process that takes a minimum of eight years. They are not required to return to their home country unless they wish to become U.S. citizens.

Conservatives call this "amnesty," but supporters like Sen. Arlen Specter, R-Pa., a lead negotiator, denies that charge.

"It's not amnesty. Amnesty is getting something for nothing. These workers have to pay a $5,000 fine. That's a pretty substantial amount for many of them," he said. Liberal critics say that the "touchback" provision, or home-country return, is not practical and would cause undo hardship on low-wage immigrants.

Conservatives also point out that the illegals, once legal, would automatically be eligible for Social Security and Medicare and would not be required to pay any back taxes, something bill supporters say is simply impossible to calculate.

Conservative Heritage Foundation President Robert Rector estimates the total cost of the bill to be in the trillions of dollars, warning, "This $2.5 trillion cost is going to come smashing into the Social Security and Medicare systems at exactly the point those systems are already going bankrupt. So the bottom line is that these individuals will make no net contribution in taxes while they are working. They will be a deficit. But when they hit retirement, they will be an astonishing cost on the taxpayer."

Unions also oppose the temporary worker program, which Dorgan and Boxer hope to rectify by stripping the program from the bill.

In a conference with reporters, both members decried what they say will be a "downward pressure" on U.S. worker wages and benefits created by hundreds of thousands of additional immigrant workers entering the system.

"It's not a compromise worthy of being called a solution to illegal immigration," Dorgan said, adding that the whole program was merely a sop to the U.S. Chamber of Commerce, which wants "cheap labor."

Boxer called the bill "unworkable" and charged that it is an "exploitation of the U.S. worker."

Separately, Democratic Sen. Chuck Schumer of New York said he will support the Dorgan-Boxer amendment because he is "fundamentally opposed to this unworkable system" whereby temporary workers can enter the U.S. without family for three two-year stints, with one year of residency outside of the U.S. in between those stints.

Immigrants coming with nuclear families get one two-year stay only and can return for an additional two years without their families.

It is unclear if the Dorgan amendment can muster the 60 votes needed for passage, but what does seem certain is that the worker program will at least be cut in half very early on with an amendment that overwhelmingly passed last year, cutting the number of work visas in half, from 400,000 per year to 200,000 per year. Boxer said she may try to reduce that number to 100,000 if her amendment fails.

Reprising a highly controversial argument last year, Inhofe's amendment to make English the official language will probably be challenged by a Democrats alternative that was supported last year that makes English merely the "common" language.

The compromise deal creates a controversial point system for earning green cards. Though the system is weighted toward families, as it is now, about 30 percent of the points will be allotted for "merit" like education level and advanced skills, something that was deemed essential by many Republicans.

Sessions said Monday the 30 percent is "not enough." He praised Canada's merit-based program and said he would work to change that in the bill.

The bill now seeks to clear the tremendous backlog of applications for citizenship within 18 months for those immigrants who applied before May 2005, a date that marks the introduction of the original Senate comprehensive immigration bill. Department of Homeland Security official say that should take care of some 567,000 applicants. Menendez said he will offer an amendment to take care of another 800,000 who applied between May 2005 and January 2007.

The price tag for this bill is expected to be extremely high, but sources told FOX News that the nonpartisan Congressional Budget Office will not have an estimate ready until after Memorial Day, something that could fracture the bill's already fragile support.

The Heritage Foundation's Rector warned that CBO will give only a 10-year estimate, "but on year 15, it starts to cost a fortune. On year 30, it will bankrupt the Social Security system. It is a disaster, it's a sham, and it's a deception."

Supporters of the bill have said the compromise is like a three-legged stool with each of the provisions — guest worker program, more permanent plan for 12 million illegals already here and border security — each being an unmovable leg, if one goes, the stool falls. Dorgan called that argument "a loose thread on a cheap suit." Boxer described the warnings as idle threats."

Republican critics like Sessions are expected to filibuster the bill, and he could receive support from a number of Democrats, like freshman Sen. Jon Tester of Montana, who on Monday, voted against allowing debate to start on the bill. The Senate permitted the beginning of a multi-week debate on a 69-23 vote to proceed with amendments.

It is very much unclear if supporters can muster the 60 votes needed for final passage, expected to come sometime around June 8, 2007. And yet another hurdle will come when the House attempts to pass a product that Speaker Nancy Pelosi, D-Calif., has already signaled will look quite a bit different from the Senate product.