Efforts to pass an immigration overhaul bill that was the result of months of work by senators, staff and administration officials fell apart Thursday, but party leaders vowed to keep trying.

Majority Leader Harry Reid, D-Nev., and Minority Leader Mitch McConnell, R-Ky., each blamed the other side for the collapse of the effort to move the legislation, but both said they thought the Senate could still pass the bill.

Others also expressed optimism that they could return to the issue in the weeks to come.

After a long day of trying to work out a deal that would allow votes on more Republican amendments in exchange for a certain time for a final vote, talks broke down and Reid held the day’s third cloture vote Thursday evening.

The result was the same as two earlier ones — bill supporters did not come close to the 60 votes necessary to cut off debate and move toward a vote on passage of the bill.

So Reid pulled the bill from the floor, pointed a finger of blame at the Republicans and called on President Bush to work harder to bring more of his GOP colleagues into line behind the bill.

Republicans have agitated all week for more time to consider more of their amendments. Reid has been equally adamant about wrapping up the debate and moving on to other business.

In a series of votes to test each side’s resolve, Republicans — even those supporting the bill — hung together for the most part.

The first cloture vote of the day, on the substitute amendment to the placeholder bill (S 1348), failed, 33-63. A second cloture vote on the bill itself failed, 34-61. The final vote Thursday night was bit closer, with some Democrats and a few GOP bill supporters returning to the fold, but it still came up well short, 45-50.

Eleven Democrats and independent Bernard Sanders of Vermont joined 38 Republicans against cloture in the final vote.

Republicans had asked Reid to delay the final cloture vote until Friday so they could continue working on a short list of amendments, according to a GOP aide. But Reid felt they had had enough time to whittle the list, and pressed on with the Thursday evening vote.

“This has been a very difficult time,” Reid said. “I think there has been a lot of bending over backwards to accommodate those who wanted to offer amendments. We really tried every possible way. . . . Really every possible way.”

McConnell said that he would work to winnow his side’s list of amendments down — earlier in the day the number was at nearly 300 — and said they could come back to the issue soon.

“Obviously it’s [Reid’s] decision,” McConnell said. “My advice is to do that some time soon.”

Losing momentum on immigration was a huge setback for Bush, who campaigned in 2000 on the issue and had reached out to Democrats on the issue after they gained control of Congress.

Bush sent two Cabinet secretaries to broker a deal, and after two months, a bipartisan group of senators produced legislation on May 17.

Homeland Security Secretary Michael Chertoff and Commerce Secretary Carlos Gutierrez made yet another trip to the Senate on Thursday in an attempt to salvage the deal, but it didn’t work.

Reid appeared to be setting up an exit strategy should immigration fade from the 110th Congress.

“This is the president’s bill and we are doing our very best to see if we can help the president,” Reid said.

For those supporting the bill, Thursday’s at least temporary defeat was tough to take, although most remained optimistic.

“I think it means we’re going to regroup,” said Lindsey Graham, R-S.C., a negotiator of the compromise. He said a small number of people on the right and left frustrated the will of the larger group. “The larger group will prevail over time,” Graham said.

But the defeat was a blow for the coalition, especially Arizona Republican John McCain, who has been defending his support of the bill while campaigning for the GOP presidential nomination.

McCain seemed distraught after the vote. Normally one to chat with reporters, he left the Senate floor without pausing, repeating variations of “I don’t have anything to say.”

Fellow Arizona Republican Jon Kyl, who helped broker the deal but voted against cloture, said “all should share some responsibility” for the failure to move the bill forward.

Edward M. Kennedy, Kyl’s counterpart in the negotiations, maintained hope that the leaders could make peace with a little time to let matters cool.

“I believe we’re well within reach of a realistic solution, and I believe we have the will to find it,” Kennedy said. “We can’t afford not to.”

Bill opponents were claiming a win.

“I think it’s a victory for sanity in this country,” said Jim DeMint, R-S.C.

Missouri Sen. Claire McCaskill was among the Democrats who voted against cloture.

She said past guest worker programs have contributed to the current illegal immigration population, with millions coming legally and overstaying visas.

“Until [the Department of Homeland Security] can demonstrate they have a handle on this problem, this bill is premature,” McCaskill said.