ld 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.

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 exposed 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.

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.

Still, they had argued that the measure, 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 an extraordinary 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