The agreement, which also mandates tougher border security and workplace enforcement, marked an extraordinary marriage of liberal and conservative goals that has the potential to bridge stubborn divides and ensure enactment of new laws this year.

Sen. Edward M. Kennedy, D-Mass., his party's lead negotiator on the deal, called it an example of the "politics of the possible," while conservative Sen. Jon Kyl (news, bio, voting record), R-Ariz., said it was the "best opportunity" for a bipartisan solution to the nation's immigration problems.

It was soon under attack, however, from a set of lawmakers and interest groups as diverse as those that united to craft it. Their varying concerns and competing agendas — along with a challenging political environment — could be enough to unravel the painstakingly written agreement.

Two of the key players in the talks from each end of the political spectrum, Sen. Robert Menendez (news, bio, voting record), D-N.J., and Sen. John Cornyn (news, bio, voting record), R-Texas, walked away from the deal before it was announced.

Conservatives branded it "amnesty," complaining that it would reward the nation's estimated 12 million illegal immigrants with a way of gaining legal status and staying in the U.S. permanently without being punished.

"What part of illegal does the Senate not understand? Any plan that rewards illegal behavior is amnesty," said Rep. Brian Bilbray (news, bio, voting record), R-Calif., chairman of the Immigration Reform Caucus.

The deal would allow illegal immigrants to come forward right away, but they could not get visas or begin a path to citizenship until the border security improvements and a high-tech worker identification program were in place.

After that, illegal immigrants could obtain a renewable "Z visa" that would allow them stay in the country indefinitely. After paying fees and fines totaling $5,000, they could ultimately get on track for permanent residency, which could take between eight and 13 years. Heads of households would have to return to their home countries first.

Liberals, on the other hand, are unhappy with the proposal because it makes a far-reaching change in the immigration system that would admit future arrivals seeking to put down roots in the U.S. based on their skills, education levels and job experience limiting the importance of family ties.

"We have concerns about the historic shift away from family unification as the backbone of our immigration system," said Kevin Appleby of the U.S. Conference of Catholic Bishops.

Immigrant advocacy and labor groups also oppose the terms of a new guest worker program in which low-skilled immigrants would be forced to leave the country after temporary stints and would have limited opportunities to stay and get on a path to permanent legalization.

"Without a real path to legalization, the program will exclude millions of workers and thus ensure that America will have two classes of workers, only one of which can exercise workplace rights," said John J. Sweeney, the AFL-CIO president.

Sen. Byron Dorgan (news, bio, voting record), D-N.D., announced Thursday that he will move to kill the guest worker program because it would hurt American workers.

Many liberal groups, which revere Kennedy as his party's decisive voice on immigration, reserved judgment on the deal, calling it a good starting point and holding out hope of improving it during next week's Senate debate. But they also voiced substantial worries.

Jorge Mursuli of People For the American Way said the measure "departs radically from America's immigration tradition of putting family reunification first. This bill also includes a future worker program that is destined for failure." Mursuli nonetheless called the plan a "solid start."

Presidential politics could also complicate the deal's chances. Fissures among the candidates started emerging swiftly after it was announced.

Sen. John McCain (news, bio, voting record), R-Ariz., who helped shape the deal, called it "long overdue." However, Mitt Romney said it was the "wrong approach," which provided "a form of amnesty" to illegal immigrants. Fred Thompson, who is considering entering the race, said it should be scrapped in favor of a measure to secure the border.

Democratic Sen. Barack Obama (news, bio, voting record), D-Ill., who is seeking his party's presidential nomination, said the proposal needed more work. "We need to fix our immigration system, but we should not replace one dysfunctional, broken system with another equally troubled system," Obama said.