President Bush and congressional leaders rushed Thursday to embrace a complicated immigration compromise that would put millions of illegal immigrants on the path to U.S. citizenship.

Even with some key details still murky, the compromise now set for a make-or-break vote Friday demonstrated crucial momentum. After feuding for more than a week, the Senate’s top Republican and top Democrat joined together to endorse the compromise outlines.

“I’m pleased that Republicans and Democrats in the United States Senate are working together to get a comprehensive immigration bill,” Bush said.

Bush was speaking in North Carolina, shortly after an extraordinary gathering of 15 senators convened in the Capitol to rally around the newest twist in the immigration debate. Crafted initially by Republicans, but given a green light by Democrats, the legalization proposal is the centerpiece of a wide-ranging immigration bill that must still be reconciled with a very different House bill.

“The good news is, we’ve had a huge breakthrough,” Senate Majority Leader Bill Frist said shortly before noon.

Frist’s Democratic counterpart, Senate Minority Leader Harry Reid, added that “we all feel good about today,” while cautioning that some loose ends still needed clarifying. California farm groups, like the Fresno-based Nisei Farmers League, and generally conservative lawmakers who hear from lots of farmers likewise assessed the newborn plan as a winner.

But even as many senators were hoping the compromise will enable them to wrap up the bill and start their two-week recess Friday night, it is inviting unanswered questions and pointed criticism. The potential for widespread fraud, a crushing administrative workload for federal border control agencies and untold expense all weigh heavily on the minds of some lawmakers.

“I’m not impressed,” said Republican U.S. Sen. Jeff Sessions of Alabama. “I’m not going to line up and say, ‘oh great, we have a compromise.’ ”

With an estimated price in the multiple billions of dollars, the overall bill stiffens border controls, provides legal status for 1.5 million illegal farmworkers and imposes new employer rules.

The single biggest controversy, though, revolves around the 11 million to 12 million illegal immigrants already in the United States. Republican senators Chuck Hagel of Nebraska and Mel Martinez of Florida authored the compromise first unveiled Wednesday night.

The proposal divides up the illegal immigrant population into several categories.

The estimated 6.7 million immigrants who have been in the United States longer than five years would get a straight shot at legal residency after meeting some requirements. They would have to pay a $2,000 fine, pay all taxes and undergo background checks, among other things.

The estimated 2.8 million illegal immigrants who have been in the United States between two and five years would face the same requirements for fines, background checks and the like. They also would have to temporarily exit the United States at a port of entry, where their fingerprints and photographs would be taken. Then, they could return to the United States with a temporary work visa and, in time, seek permanent legal status.

“I think we’ve reached a plea bargain with (the) illegal immigrants,” said Republican U.S. Sen. Lindsey Graham of South Carolina.

The 1.6 million illegal immigrants who have been in the United States for less than two years, according to Pew Hispanic Center estimates, would have to leave the country and apply through normal, time-consuming channels for visas.

“You are going to have a number of people who are not going to make it,” conceded Democratic U.S. Sen. Edward Kennedy of Massachusetts. “That’s the reality of it.”

The compromise doesn’t explain why any of the 1.6 million illegal immigrants who have been in this country for less than two years would voluntarily leave the United States. The bill places its big enforcement emphasis along the Mexican and Canadian borders, where senators call for doubling the number of Border Patrol agents and development of a “virtual fence” that includes lights, cameras and unmanned aerial vehicles.

The compromise plan is set for a vote Friday morning where it will need 60 votes to prevail and avoid a filibuster. An unknown number of amendments will also be offered. On Thursday morning, on a mostly party line vote, Democrats failed by a 39-60 margin in their effort to move a different version of the immigration bill.