The future looked grim for an immigration overhaul legislation late Wednesday, with several key senators weighing objections that could kill the issue for the 110th Congress.

The crucial procedural step comes Thursday morning, when senators vote on whether to limit debate on the bill (S 1639). Supporters were scrambling to find the 60 votes needed to get to a vote on final passage, which would come Friday if cloture is invoked.

If they fail, as some are predicting, attempts to deal with immigration could be lost until after the 2008 election.

Though 64 senators agreed June 26 to proceed to the bill, more than a dozen said they did so only to allow debate on 27 amendments that they hoped would modify core provisions they found objectionable.

“It’s tight,” said California Sen. Dianne Feinstein, a key Democratic supporter of the legislation.

The list of potential “no” votes on cloture now includes New Mexico Republican Pete V. Domenici and New Jersey Democrat Robert Menendez — both of whom voted in favor of proceeding to the bill two days ago. Menendez failed in an effort to modify the bill Wednesday.

Perhaps the most telling vote will come from Nebraska Democrat Ben Nelson, who has made a practice of supporting cloture motions whether he intends to vote for the legislation or not. Nelson said Wednesday he would likely break that tradition and vote against ending debate on the immigration bill.

Before the Senate voted Wednesday on an amendment he offered, Menendez said his vote on cloture depended on the outcome of his and other amendments.

“I think right now, my sense is they are short of cloture votes, and I hope they are willing to make some accommodations,” Menendez said.

The chamber later voted, 55-40, to table his amendment, which would have placed more weight on family ties for issuing green cards in a new merit-based point system.

Domenici said he was “leaning” against cloture, indicating that the bill’s language remains a moving target and that even if the Senate passes the measure, he doubts it will go much further.

“I’m against the bill,” said Domenici. “The more we’ve had amendments offered and had to clarify, the more I have concerns about whether it can be enforced. My concern about whether it becomes law is growing.”

Feinstein, who remained hopeful that enough votes were in place, questioned Domenici’s rationale.

“If you don’t stand up to be counted, nothing ever becomes law,” she said. “You have to start somewhere.”

Nelson, Domenici and Menendez are not alone. A number of Republicans and Democrats, for different reasons, have questioned aspects of the legislation that would allow millions of illegal immigrants to stay, receive legal status and ultimately earn citizenship; provide $4.4 billion in mandatory spending for border security and enforcement; and create a new temporary worker program.

Several said in interviews that they were leaning toward voting against cloture, including some who supported cloture on the motion to proceed to the bill on June 26.

Democrats have expressed dissatisfaction with the guest worker program and an effort to reduce the role family relations play in awarding green cards.

Others in both parties, but particularly Republicans, have questioned the legalization provision and past enforcement promises that have yet to be fulfilled.

The list of Republicans who say they are contemplating switching their votes includes Norm Coleman of Minnesota, John Ensign of Nevada, Gordon H. Smith of Oregon and Christopher S. Bond of Missouri.

Ensign said he was hearing that Democrats were redrafting his amendment — as part of the larger “clay pigeon” amendment package the text was no longer his to control • — in a way that would make it non-germane after cloture. His amendment would deny Social Security benefits earned by illegal immigrants prior to becoming legalized.

“I may vote against cloture anyway, but I will definitely vote against cloture if I don’t get a vote on my amendment,” Ensign said.

Democrats create more challenges for vote counters, because they might be switching in either direction.

Some, like Jeff Bingaman of New Mexico, Barbara Boxer of California, Jim Webb of Virginia and Mark Pryor of Arkansas, voted against cloture on a previous version of the bill (S 1348) but agreed to resume the debate this week.

Then there are those such as Debbie Stabenow of Michigan and Evan Bayh of Indiana who voted for cloture June 7 but against resuming debate this week.

Stabenow said she would “probably” vote against cloture Thursday.

Senators posing the most danger to the bill’s future, though, are those who have supported cloture each time but are now ready to abandon the process.

Menendez was at one time a key player in drafting the legislation. He participated in the majority of meetings behind the scenes with Republicans, Democrats and members of the Bush administration.

But he walked away from the negotiations hours before the first draft became public, and he has failed to get any amendments adopted that answer his concerns about family reunification.

“It’s been a one-way street, and that dog won’t hunt,” Menendez said.

Several were equally incensed by the clay pigeon procedural tactic employed by Senate Majority Leader Harry Reid, D-Nev., and the bipartisan backers of the legislation to speed consideration of the amendments.

The mood shifted as the bipartisan group that negotiated the measure, the so-called grand bargainers, used the tactic to rebuff the first several amendments in a series of more than two dozen.

But the process came to a halt when the chamber refused to table an amendment by Max Baucus, D-Mont., that would remove all references in the legislation to requirements for issuing secure driver’s licenses and identification cards under a 2005 law (PL 109-13).

The individual amendments that make up the clay pigeon can be adopted only at the end of debate on the bill, except in the unlikely event they are agreed to by unanimous consent. Such consent is next to impossible with so many critics lined up threatening to stall the process.

So senators began voting Wednesday on whether to table the individual amendments. If an amendment is tabled, that element of the omnibus clay pigeon is gone.

However, when the chamber voted against tabling the Baucus amendment, the process stopped. Without a unanimous consent agreement to adopt the amendment or move on, the next step was the cloture vote.

“It’s definitely a Rubik’s Cube,” said Maria Cantwell, D-Wash., who believes the grand bargainers will squeak by on Thursday. “I think they always thought it would be close.”