Despite the unwavering optimism of the bipartisan sponsors of a sweeping immigration bill, both Democrats and Republicans said the rancorous politics and complex policy issues associated with the measure — not to mention the Senate’s packed summer schedule — may be too insurmountable to salvage a compromise.

Senate Majority Leader Harry Reid (D-Nev.) promised to revisit the bill even though 38 Republicans and 12 Democratic-leaning Senators — 50 in all — voted to block the bill last Thursday night, but Reid conditioned his willingness to bring the bill back up on whether the bipartisan group could come up with a short list of amendments and a guaranteed way to get the bill to a final vote in a short period of time.

Bipartisan negotiators vowed on Friday to craft just such an agreement with the conservative Republicans who led the filibuster of the bill, saying they believed they could come up with a deal that would only take three more legislative days. But both Republicans and Democrats cast doubt on their ability to bridge the divide that stymied the bill last week.

“Of course the negotiators are going to say that because they have the most invested in it,” said Sen. John Thune (R-S.D.), who was one of several conservatives agitating to have votes on their amendments. “I think that’s very optimistic.”

Thune said Reid’s decision to press forward on a vote to end debate, or invoke cloture, on Thursday eliminated all the leverage he and the negotiators had with conservative dissenters.

“There was some leverage there that was lost. I think it’s going to be hard to get Members back to the table,” Thune said.

Indeed, sources indicated that conservatives such as Sens. Jim DeMint (R-S.C.) and Jeff Sessions (R-Ala.) may be emboldened by their ability Thursday to rally GOP troops to oppose cloture on the bill.

“The majority of Republicans were united and stay united in the right of the minority to have full and fair debate,” said one knowledgeable Senate GOP aide. “The thought that we’re going to restart this process and all of the sudden do this over again in three days is not reality.”

GOP negotiators, however, said they anticipate that more Republicans will abandon Sessions and DeMint as the debate goes forward. Several GOP sources indicated that Minority Leader Mitch McConnell (R-Ky.) was on the verge of delivering several leadership votes, along with several rank-and-file Republicans, for cloture — thus for keeping the bill alive — until Reid pressed ahead with the vote Thursday night.

“Some of our Republican colleagues are going to have to realize that the [rest of the Senate Republican Conference] will not stay with them,” said Sen. Arlen Specter (R-Pa.). “There comes a breaking point.”

But there was little consensus last week on exactly how to go about getting the bill to the finish line.

Sen. Bob Menendez (D-N.J.), a critic of the bill but one of a majority of Democrats who voted for cloture to end debate, said in an interview Friday that the measure will never go anywhere unless more Republicans come on board. That will happen only if President Bush personally kick-starts a massive lobbying campaign within his ranks, he said.

“The bill will see the light of day if Bush puts whatever muscle he has left behind it and gets Republicans to join us outside of the seven people who joined us yesterday,” Menendez said. “If he can’t get another 10 votes, then it’s not a bipartisan bill.”

But Republicans largely rejected the notion that Bush would be able to make a difference, even with the president scheduled to visit the Senate Republican Policy Committee lunch on Tuesday.

“The president made his case very vigorously already to the people who are gettable,” Thune said.

Even if Bush were to make another plea for the bill this week, Thune said Senators already have heard too much opposition from their constituents.

“I don’t think it’s really going to change a lot of minds,” Thune said. “A lot of these opinions are starting to harden.”

Others, however, said Bush could perhaps influence some Senators if he lobbied them directly, rather than through his two key surrogates, Commerce Secretary Carlos Gutierrez and Homeland Security Secretary Michael Chertoff.

“He has to do the one-on-ones,” said a senior GOP Senate aide. “That’s where the convincing comes.”

Though Bush’s appearance at Tuesday’s GOP policy lunch was in the works before Thursday’s failed cloture vote, it almost certainly will focus on the future of the immigration plan. And administration officials held a conference call with several key GOP Senators on Friday morning to talk about their strategy.

Still, Thune put the onus on Democrats to make up the 15-vote deficit on cloture, saying Republicans had nearly maxed out support on their side.

“I think the Democrats are going to have to deal with most of those,” he said.

But Menendez said Republicans are going to have to meet Democrats halfway, and only then would a share of those 12 members of the Democratic Caucus who opposed cloture come on board.

“We might be able to turn some Democrats if it means we can ultimately get to cloture,” the New Jersey Democrat said. “But if people see we’re not going to get cloture, why risk taking that vote?”

Meanwhile, Reid has the hairy task of finding an opening to schedule immigration again this summer. The month of June already is expected to be eaten up by at least two weeks of debate on energy policy and another week on the Defense Department authorization, which would take the Senate into the July Fourth recess. Meanwhile, Reid has set aside the bulk of the month of July to work on the annual spending bills to make good on his pledge to give appropriations measures ample floor time before the start of the new fiscal year Oct. 1.

“The Majority Leader has already made his intentions clear,” said a senior Republican Senate aide who cast doubt on whether the immigration debate would return given Reid’s crowded schedule.

In fact, before the bill collapsed Thursday, Senate Majority Whip Dick Durbin (D-Ill.) pointed to the Senate schedule, and appropriations bills in particular, as a real problem if cloture failed.

“Harry Reid is in a terrible position,” Durbin said.

Steven T. Dennis contributed to this report.