WASHINGTON, D.C. -- The U.S. Senate has voted to revive broad legislation to overhaul the nation's immigration system, after the measure was blocked by opponents earlier this month. But the fate of the measure remains uncertain.

The Senate Tuesday voted 64 to 35 to revive the bill.

Under Senate rules, at least 60 votes were necessary in the 100-seat chamber to move the bill forward.

The Senate action came shortly after President Bush urged lawmakers to support the legislation.

"I view this as an historic opportunity for Congress to act, for Congress to replace a system that is not working with one that we believe will work a lot better," said Mr. Bush.

The measure would tighten border security, create a temporary guest worker program and grant immediate legal status to millions of undocumented workers in the United States.

The bill was crafted by a group of Republican and Democratic senators and the White House after months of negotiations.

President Bush has made immigration reform a top domestic priority. He and his aides have been intensely lobbying fellow Republicans on Capitol Hill to support the bill after Senate opponents blocked it from coming to a vote earlier this month.

The Senate's top Democrat, Majority Leader Harry Reid of Nevada, says Republican support will be crucial to getting the bill passed in the chamber, where Democrats hold a slim majority.

"We have an immigration system that is broken and needs to be fixed," he said.

"That is what we are trying to do, is fix this. We would be derelict in our duties if we did not make every effort to get the legislation passed."

But many Republicans question whether the bill will go far enough in securing U.S. borders and they argue it will reward immigrants who came to the United States illegally with the possibility of U.S. citizenship.

Senator Jeff Sessions, an Alabama Republican, is a leading opponent of the legislation.

"The bill is flawed," he said.

"It will not work."

Sessions and other opponents say their efforts to scuttle the legislation are gaining momentum.

Prospects for the bill's passage remain unclear, with another test vote scheduled Thursday.

In addition, several proposed amendments, if passed, could alter key parts of the legislation, potentially threatening the fragile coalition supporting the bill.