Washington ? The Senate handed President Bush a devastating defeat on his top domestic priority Thursday by blocking far-reaching immigration legislation to enable as many as 12 million undocumented immigrants to remain in the country legally.

After more than a month of off-and-on debate, the Senate balked at a crucial procedural motion to begin advancing toward a final vote by late tonight. The motion was defeated 46-53, far short of the needed 60-vote supermajority of the Democratic-controlled, 100-member Senate.

Although the bill was resurrected after the defeat of two similar motions last month, the latest Senate vote may have marked the epitaph for legislation that Bush and his allies deemed as vital to repairing the nation’s chaotic immigration system and providing legal safeguards to the millions of illegal immigrants who have come into the country during the past two decades.

The defeat also underscored Bush’s declining popularity and withering political capital within his own Republican Party. A highly vocal cadre of conservative Republicans – including the two senators from his home state of Texas – rebelled against the bill, assailing the legalization provision as amnesty that rewarded illegal behavior.

Opponents also included liberal and moderate Democrats who feared that a proposed temporary guest-worker program would hurt wages and jobs for American workers. Some also worried that a new point-based merit system for future immigrants would undercut family-based immigration, a cornerstone of the nation’s current immigration system.

Lawmakers on both sides say it could be years before Congress is willing to tackle the issue again, particularly with the onset of the 2008 presidential campaign.