WASHINGTON - A broad immigration bill to legalize millions of people in the U.S. unlawfully suffered a stunning setback in the Senate Thursday, costing President Bush perhaps his best opportunity to win a top domestic priority.

The bipartisan compromise championed by the president failed a crucial test when it could not attract even a simple majority for an effort to speed its passage.

Intense public concern over immigration across the country conspired with high political stakes to produce a roiling debate on the issue. Ultimately, those forces overwhelmed a painstakingly forged liberal-to-conservative alliance that sought to insulate their compromise from partisanship.

Supporters could muster only 45 votes to limit debate and speed the bill to final passage, 15 short of what was needed on the procedural maneuver. Fifty senators voted against cutting off debate.

Most Republicans voted to block Democrats' efforts to advance the measure.

Senate Majority Leader Harry Reid, D-Nev., who had made no secret of his distaste for parts of the bill, quickly pulled it from the floor and moved on to other business, leaving its future uncertain.

He insisted that the bill was not dead, but a crowded Senate calendar complicates its prospects.

"I, even though disappointed, look forward to passing this bill," Reid said. "I have every desire to complete this legislation, and we all have to work - the president included - to figure out a way to get this bill passed."

The measure's chances are even murkier in the House, where Democratic leaders don't plan to act on the divisive issue until the Senate has finished work on it.

Sen. Mitch McConnell, R-Ky., the minority leader, said Democrats tried to rush the bill.

"I think we're giving up on this bill too soon," McConnell said.

The legislation would tighten borders and institute a new system to prevent employers from hiring undocumented workers, in addition to giving up to 12 million illegal immigrants a pathway to legal status.

Conceived by an improbable coalition that nicknamed the deal a "grand bargain," the measure exposes deep rifts within both parties and is loathed by most GOP conservatives.

All but seven Republicans voted against ending debate, with many arguing they needed more time to make the bill tougher with tighter border security measures and a more arduous legalization process for unlawful immigrants. Thirty-eight Republicans and Sen. Bernard Sanders, a Vermont independent, opposed the procedural tactic.

All but 11 Democrats supported the move, but they, too, were holding their noses at provisions of the bill. Many of them argued it makes second-class citizens of a new crop of temporary workers and rips apart families by prioritizing employability over blood ties in future immigration.

Thirty-seven Democrats and Sen. Joseph Lieberman, a Connecticut independent, voted to advance the measure.

Proponents had argued that the bill, on balance, was worth advancing.

"We can all find different aspects of this legislation that we differ with," said Sen. Edward M. Kennedy of Massachusetts, the leading Democratic architect of the bill.

He held out hope after the vote that the measure would survive. "Doing nothing is not an alternative," Kennedy said. "This issue isn't going away."

"I believe that we will yet succeed," said Sen. Arlen Specter, R-Pa., a framer of the bill who was one of few in his party who backed the procedural move.

The defeat for the compromise was the culmination of a week of ups and downs for the contentious immigration measure, which mirrored the tumultuous process that went into crafting it.

Kennedy partnered with Sen. Jon Kyl, R-Ariz., and several centrists to craft a bill that melded conservative themes of tougher border security and limiting immigration with the liberal goal of legalizing those who are in the U.S. unlawfully and welcoming future arrivals.

In the end, however, Kyl broke from the bipartisan clique that hatched the agreement, siding with Republicans who said they hadn't gotten enough chances to toughen the bill.

"It's time to scrap this mess of a bill," said Sen. Jim DeMint, R-S.C., a conservative who had failed in several attempts to make the measure more punitive toward illegal immigrants.