Washington ? A broad immigration bill to legalize millions of people living unlawfully in the U.S. failed a crucial test vote 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 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.

Senate Majority Harry Reid, D-Nev., who had made no secret of his distaste for parts of the bill, said earlier he would move on to other matters if the immigration measure’s supporters didn’t get 60 votes Thursday night.

The defeat set off a bitter round of partisan recriminations, with Democrats and Republicans each accusing the other of killing it.

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.

Reid pulled the bill from the floor and moved on to other business, costing the measure perhaps its best chance at enactment. He insisted that the immigration bill is not dead for the year. “I, even though disappointed, look forward to passing this bill,” he said.