WASHINGTON, June 7 –The Senate refused at midday to shut off debate on the immigration overhaul bill and move toward a vote, leaving the fate of the legislation uncertain and setting up another, all-important procedural vote this evening.

The move to end debate was rejected by 63 to 33, so the bill’s backers fell 27 votes short of the 60 needed to invoke what is known as cloture and set up a yes-or-no vote on the legislation itself.

The result was a setback not only for the bill’s supporters but also for President Bush, who has made a comprehensive immigration bill one of his top legislative priorities.

Nevertheless, Senator Harry Reid of Nevada, the Democratic majority leader, scheduled another, make-or-break cloture vote for this evening. If that vote also falls short, Mr. Reid is expected to shelve the bill, meaning that changes in immigration law might not be considered again for many months.

The midday move to end debate failed chiefly because a significant number of conservative Republicans wanted more time to offer amendments to make the measure more to their liking.

Some 12 hours before the noontime cloture vote, the bill’s supporters suffered a setback when the Senate voted to put a five-year limit on a new guest worker program that would be created under the legislation. By a vote of 49 to 48 shortly after midnight, the Senate approved the limit, in the form of an amendment by Senator Byron L. Dorgan, Democrat of North Dakota.

The temporary worker program is an important element of the “grand bargain” on immigration forged in three months of negotiations by a small group of senators from both parties.

If the Senate votes this evening to end debate, the bill will have cleared a major hurdle — but by no means the last one. The House has yet to take up its version of the immigration legislation, and the issue has deeply divided the representatives. Many conservatives want to do more to restrict immigration and to toughen border enforcement. Many liberals, including members of the Congressional Hispanic Caucus, want to do more to protect immigrants’ rights and promote family-based immigration. The Senate bill, which embodies a fragile compromise strongly supported by the president, would offer most of the estimated 12 million illegal immigrants in the United States the chance to obtain legal status. It calls for the biggest changes in immigration law in more than two decades.

Supporters contend that it would address the problem of millions of illegal aliens without giving them amnesty; that it will further secure the nation’s borders, and that through its guest-worker program it will help immigrants and American employers. Its opponents have argued that there are far too many deficiencies in its nearly 400 pages.