WASHINGTON - It took an improbable alliance of senators from the hard right and far left to strike a sweeping immigration deal to fix problems that have bedeviled Republicans and Democrats for decades.

The coalition faces a daunting test of its strength next week as the Senate begins debating the plan unveiled Thursday, one which would grant legal status to millions of people in the country unlawfully and fulfill a top domestic goal of President Bush.

Bush said the proposal would “help enforce our borders but equally importantly, it'll treat people with respect.”

A carefully crafted hybrid of conservatives' and liberals' divergent philosophies on immigration, the deal also mandates bolstered border security and a high-tech employment verification system to prevent illegal workers from getting jobs.

It was sealed after months of secretive bargaining between the White House and lawmakers in both parties - from liberal Democrat Sen. Edward M. Kennedy of Massachusetts to conservative Republican Sen. Jon Kyl of Arizona - on an issue that carries heavy potential risks and rewards for all involved.

Senate Majority Leader Harry Reid, D-Nev., said debate would begin on Monday, but he cautioned, “I don't know if the immigration legislation is going to bear fruit and we're going to be able to pass it.”

Almost instantly, the plan brought vehement criticism from both sides of the immigration issue, including liberals who called it unfair and unworkable and conservatives who branded it an overly permissive “amnesty.”

The proposal constitutes 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. A new class of guest workers would be allowed in temporarily, but only after the new security measures were in place - expected to take 18 months.

Kennedy hailed it as “the best possible chance we will have in years to secure our borders and bring millions of people out of the shadows and into the sunshine of America.”

Kyl said the measure wasn't perfect, “but it represents the best opportunity that we have in a bipartisan way to do something about this problem.”

It was clear, however, that many Republicans and Democrats were deeply skeptical. Reid said it needed improvement.

“I have serious concerns about some aspects of this proposal, including the structure of the temporary worker program and undue limitations on family immigration,” Reid said.

In a reminder of the delicate nature of the coalition, some lawmakers on both ends of the political spectrum who attended the weeks of closed-door talks that yielded the agreement deserted it at the last moment.

Sen. Robert Menendez, D-N.J., said the proposal “tears families apart” because a new point system used to evaluate future legal immigrants would value family connections well below employment-related criteria.

“When you anchor yourself to the far right and you give, I think, relatively little, it's hard to meet the challenge” of producing a workable bill, Menendez said in an interview.

Sen. John Cornyn, R-Texas, said he had “very serious concerns with the principles outlined” in the agreement.

And conservatives on both sides of the Capitol derided the deal as “amnesty” for illegal immigrants, using a politically charged word that figured prominently in campaigns across the country last year.

“I don't care how you try to spin it, this is amnesty,” said Sen. Jim DeMint, R-S.C.

The proposed agreement would allow illegal immigrants to come forward and obtain a “Z visa” and - after paying fees and a $5,000 fine - 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.

They could come forward right away to claim a probationary card that would let them live and work indefinitely in the U.S., but they could not seek citizenship until border security improvements and the high-tech worker identification program were completed.

A new crop of low-skilled guest workers would have to return home after stints of two years. They could renew their visas twice, but would be required to leave for a year in between each time. If they wanted to stay in the U.S. permanently, they would have to apply under the point system for a limited pool of green cards.

The program drew fire from liberal groups that said it was unworkable. They had joined Democrats in pressing instead for guest workers to be permitted to stay and work indefinitely in the U.S., and ultimately earn the chance to stay.

“Without a clear path to permanent residence for a healthy share of the future temporary workers, we run the risk of reproducing the widespread illegality that this bill is designed to address,” said Frank Sharry, the executive director of the National Immigration Forum.

Sen. Byron Dorgan, D-N.D., said he would try to kill the temporary worker program because it would bring in a potentially unlimited stream of immigrants to compete with Americans for jobs and depress wages.

In perhaps the most hotly debated change, the proposed plan would shift from an immigration system primarily weighted toward family ties toward one with preferences for people with advanced degrees and sophisticated skills. Republicans have long sought such revisions, which they say are needed to end “chain migration” that harms the economy.

Family connections alone would no longer be enough to qualify for a green card - except for spouses and minor children of U.S. citizens. Strict new limits would apply to U.S. citizens seeking to bring foreign-born parents into the country.

The issue quickly became a subject of debate among presidential candidates in both parties, exposing divisions among Republicans.

Sen. John McCain, R-Ariz., who led the charge last year to push through an immigration overhaul, called the deal “the first step” and urged moving it forward before the politics of 2008 made such action impossible.

“We all know that this issue can be caught up in extracurricular politics unless we move forward as quickly as possible,” said McCain.

Mitt Romney, another Republican presidential hopeful, issued a statement calling the plan “the wrong approach,” saying it conferred “a form of amnesty” on illegal immigrants. “That is unfair to the millions of people who have applied to legally immigrate to the U.S.,” the former Massachusetts governor said.

Former Republican Sen. Fred Thompson of Tennessee, who is weighing a presidential bid, said the measure should be scrapped in favor of one that secures the border.

“With this bill, the American people are going to think they are being sold the same bill of goods as before on border security,” Thompson said in a statement.

Democratic Sen. Barack Obama of Illinois said the compromise needs work.

“Without modifications, the proposed bill could devalue the importance of family reunification, replace the current group of undocumented immigrants with a new undocumented population consisting of guest workers who will overstay their visas, and potentially drive down wages of American workers,” Obama said in a statement.