Lawmakers want to revise key elements, such as letting millions of illegal immigrants stay in the U.S., favoring skills and education over families and setting out the terms of a new temporary worker program.
Any one of the changes has the potential to sink the whole measure, which was unveiled with fanfare Thursday but was still being drafted late Friday.
Commerce Secretary Carlos Gutierrez, who helped negotiate the compromise, called it "very well-balanced," and cautioned against revisions that could upset the framework.
"You take something out and you're creating a problem throughout the system - you may think that you're only tweaking one part," Gutierrez said in an interview. "We've got to be very careful as to what is proposed to change."
As the White House and proponents in both parties began laboring to sell the agreement to the public, interest groups launched elaborate efforts to alter major pieces of the complicated proposal.
"We're going to fight like mad to fix the parts we don't like," said Tom Snyder, the national political director of UNITE HERE!, a service workers union comprised largely of immigrants.
Senate leaders huddled privately to plot strategy for next week's debate, which is likely to feature Democratic efforts to kill or substantially shrink the temporary worker program and Republican attempts to prevent illegal immigrants from staying indefinitely in the U.S. without applying for permanent residency or citizenship.
Majority Leader Harry Reid, D-Nev., has said he doesn't know whether the measure can make it through the Senate.
Liberal activists who call the measure a good start but object to major parts have "a couple of bites at the apple" to change it as it makes its way to President Bush's desk, said Frank Sharry, the executive director of the National Immigration Forum.
"It's been hatched in the backroom at the eleventh hour; it's now going to be debated in the light of day," Sharry said.
If they don't succeed in reshaping it, the groups that have been among the strongest proponents of an immigration overhaul might desert the deal.
"We're not sure that our support for moving forward will continue to be support if the bill that approaches the finish line has these kind of problems in it," said Cecilia Munoz of the National Council of La Raza.
There's broad agreement on some elements of the plan, such as improving border security and workplace enforcement and allowing some way for the nation's 12 million illegal immigrants to earn a path to citizenship.
But the plan's cornerstones are among its most controversial elements.
It would allow the millions of undocumented immigrants already here to gain legal status virtually immediately and keep it indefinitely through a new "Z visa" that could be renewed repeatedly. Those seeking permanent legal residence or eventual citizenship would face long waits and have to pay fees and fines, and their household heads would have to return to their home countries.
Some conservative Republicans want to limit the amount of times Z visas could be renewed.
The deal also proposes a fundamental reordering of the nation's immigration priorities, moving the system from one based on family to one primarily designed to meet the needs of U.S. employers. While spouses and minor children of legal residents and citizens could still get green cards under the new system, other relatives would have to qualify under a point system that rewards advanced skills, education, English proficiency and experience in high-demand occupations.
This has drawn fire from both sides, with conservatives arguing that the system should not reward low-skilled workers or extended family at all, and liberals saying it devalues family.
"The merit-based point system that was promised is just a shell of what it could have been and what it should have been," said Sen. Jeff Sessions, R-Ala., who wants to guarantee that a larger percentage of future green cards are awarded purely based on employment criteria.
On the other side, Sen. Hillary Rodham Clinton, D-N.Y., plans to offer an amendment that would exempt the spouses and children of lawful permanent residents from the measure's visa caps, guaranteeing that families receive a higher priority.
The guest worker program, which would provide 400,000 visas yearly for immigrants seeking temporary employment, has come under criticism from conservatives and populists who think it's too expansive and immigrant advocacy groups who say it creates a working underclass with few rights. Sen. Byron Dorgan, D-N.D., wants to strike the temporary worker program, arguing that it harms American workers and depresses wages. Workers could come for two-year stints and renew their visas twice, with a year home in between each time, but would ultimately have to qualify for green cards based on the point system. The system "is skewed against them," La Raza's Munoz said. "We don't consider that to be a meaningful path to citizenship." The Bush administration, cognizant of the deal's vulnerability, is already toiling to line up the votes needed to push it through Congress. "We're going to work as hard as we need to," Gutierrez said. AP-CS-05-18-07 1754EDT