The compromise brought liberal Democrats and conservative Republicans together with top members of President Bush's team on an issue that carries heavy potential risks and rewards for all involved.
The Senate will wade into an emotional and wide-ranging debate on the issue next week that promises to test the unlikely coalition that produced the deal. 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 borders were fortified and measures were in place to ensure the rules were followed.
Bush said the proposal would "help enforce our borders but equally importantly, it'll treat people with respect."
"This is a bill where people who live here in our country will be treated without amnesty but without animosity," Bush said.
The formula was enough to satisfy liberal Sen. Edward M. Kennedy, D-Mass. - viewed as his party's decisive voice on immigration - and Sen. Jon Kyl, R-Ariz., a conservative who has been adamantly opposed to past overhauls.
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. Senate Majority Leader Harry Reid, D-Nev., called the proposal a "starting point" for next week's debate and 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 alliance, 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 legally in the U.S., but could not begin the path to permanent residency or 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.