Activists gather for an immigration rally outside the US Capitol 02 June 2007 in Washington, DC. A bill on granting legal status to 12 million illegal immigrants failed a key test in the US Senate Thursday, killing its best chance of passage and dealing a blow to President George W. Bush. [File Photo: AFP]

The U.S. Senate on Thursday failed in a crucial vote on an immigration bill that could provide a path toward legal status to the estimated 12 million illegal immigrants in the country.

The Senate voted 45-50 against limiting debate on the legislation, which was supported by President George W. Bush and would tighten border security.

The bill would also punish employers for hiring illegal immigrants, establish a merit-based points system for future immigrants, and included an amendment that would discontinue a guest worker program within five years.

Most Republicans voted against limiting debate on the bill, while most Democrats supported the move.

The vote was 15 short of the 60 needed in the 100-seat Senate for the bill to move on.

As Senate Majority Harry Reid said before the vote that he would move on to other matters if proponents for the legislation did not get the necessary 60 votes Thursday night, the future of the bill has become uncertain.

"There's lots of support for this program on the outside," said Reid, "The problem was on the inside of this Senate chamber."

"I want to finish this bill, but I can't do it alone," the Democrat from Nevada added, "Let's have President Bush work with us."

As the Senate moves on to other issues, the chance of passing and enacting the bill before the next administration takes office looks very slim.

Senate Republican leader Mitch McConnell said he hoped the Senate would "return to this issue in the near future. I doubt prospects would get better with the passage of time."

Reid had appealed to Bush to help persuade 47 Republicans who opposed limiting debate on the bill in an earlier vote on Thursday.

The legislation's demise would produce headlines that "the president fails again," the majority leader warned.

Bush has championed an overhaul of immigration laws since taking office six years ago. The vote is a defeat for him.

The Senate last year approved a measure creating a guest-worker program and a path to legal status for undocumented immigrants.

However, negotiations with the House of Representatives failed to produce a final bill.

"The vote was obviously a big disappointment, but it makes no sense to fold our tent, and I certainly don't intend to," Democratic senator Edward Kennedy, a chief supporter of the bill, said in a statement.

The Senate debated the legislation for nine days and considered more than three dozen amendments.

However, the measure eventually stalled in the Senate after Republicans sought more opportunities to alter the bill.