A broad immigration bill to legalize millions of people unlawfully in the United States failed a crucial test vote in the Senate Thursday, a stunning setback that could spell its defeat for the year.

The vote was 45-50 against limiting debate on the bill, 15 short of the 60 that the bill's supporters needed to prevail. Most Republicans voted to block Democrats' efforts to bring the bill to a final vote.

The legislation, which had been endorsed by President Bush, would tighten borders, institute a new system to prevent employers from hiring undocumented workers and give as many as 12 million illegal immigrants a pathway to legal status.

Senate Majority Harry Reid, D-Nevada, who had made no secret of his distaste for parts of the bill, said he would withdraw it but keep working toward eventual passage.

"I, even though disappointed, look forward to passing this bill," Reid said. But he said he needs help from the White House.

"This is the president's bill," Reid said. "... We can't do it alone over here. We need some help."

Senate Minority Leader Mitch McConnell, R-Kentucky, urged Reid not to give up on the legislation, but allow debate on amendments to continue in order to resolve the objections of conservative senators.

"This is a complicated bill, but the key to passage is not complicated," McConnell said.

McConnell said all that's needed is a "reasonable number of additional roll-call votes" on amendments.

"We're not that far away from being able to get cloture on the bill," he said.

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.

Most Republicans voted against ending debate, saying 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 a handful of 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.