WASHINGTON–Efforts to repeal the Pentagon’s ban on homosexuals serving openly in the military hit a fresh Republican blockade in the Senate Thursday but proponents, including Sen. Joseph Lieberman, say they have a fall-back plan that could revive the bill before the end of the lame-duck session.

In a surprising and dramatic Senate vote, supporters of repealing the military ban, known as “Don’t Ask, Don’t Tell,” fell three votes short of the 60 needed to overcome a threatened GOP filibuster. The final tally of 57-to-40 included only one Republican, Sen. Susan Collins, R-Maine, voting in favor of taking up the bill.

The result thwarted action not only on repealing “Don’t Ask, Don’t Tell,” but also on a sweeping defense authorization bill. The repeal is included in that broader measure.

The timing of the vote, if not the outcome, took advocates by surprise, since Lieberman, an independent from Connecticut, and Collins were deep in negotiations with Senate Majority Harry Reid, D-Nev., on an agreement that they said would have almost certainly guaranteed several more GOP votes in favor of starting debate on the legislation.

“This afternoon, process triumphed over those principles” that dictate an end to the discriminatory policy, Lieberman said. “As a result, we have suffered a setback.”

But he and Collins said they had a new strategy: stripping the DADT language out of the broader defense bill and pushing it as a stand-alone measure. And they said Reid had assured them they would get a vote on that slimmed-down bill before the end of the year.

“I am convinced that there are 60 or even 61 or 62 votes to repeal ‘Don’t Ask, Don’t Tell’ under this carefully constructed language,” Collins said.

A stand-alone bill would still be subject to filibuster and possibly other procedural stumbling blocks. “We’re not kidding ourselves–this is not going to be easy,” Lieberman said. “On the other hand, if you have a free-standing bill… it’s going to be a much more limited debate.”

If it does clear the Senate, it would also still have to win approval in the House, although that chamber is already on record supporting repeal of DADT. And President Barack Obama has called for Congress to nix the policy.

“I hope it will be one of the remaining priorities” of the lame duck session, Lieberman said.

Still, he and Collins acknowledged disappointment in Thursday’s untimely defeat.

Republicans have said they would block all legislation until the Senate acts on a measure to renew the Bush-era tax cuts.

Lieberman and others had argued in private negotiations that there would still be time to take up the defense authorization bill, with the DADT repeal included, after that had been dispensed with. And he and Collins were working on a deal that would have allowed debate on 15 or so amendments to the underlying defense bill, including one to strike out the DADT language.

They said they had the votes to win on those terms.

“I guess we lost the argument with Sen. Reid,” Lieberman, who caucuses with the Democrats, said at a news conference after the vote. He said Democrats were operating under a false notion that they had to adjourn for the year by Dec. 17.

In the meantime, however, he argued “we are wasting time” on a gamut of other bills that are virtually certain to end up in the legislative dustbin–not on the President’s desk.

“I’m perplexed and frustrated, given the progress we had made, that [Reid] called the vote so suddenly,” Collins said. “There was a clear path to victory and for the life of me, I can’t understand why the majority leader failed to take it.”

Sen. Lisa Murkowski, R-Ala., who supports repealing DADT, was among those who stuck with her party to block taking up the bill on Thursday.

“Nothing was offered in terms of a reasonable process,” said Murkowski. Reid “shut it down.”

Before he called the vote, Reid said on the Senate floor that he’d done everything he could to work out a reasonable process but that Republicans, other than Collins, were being intransigent.

“I have bent over backwards to find a way to get this bill done. But it is clear that Republicans–led by a couple of senators who simply do not want to have a vote on repealing ‘Don’t Ask, Don’t Tell’– are doing everything they can to stand in the way,” Reid said.

“They want to block a vote on this issue at all costs, even if it means we do not pass a Defense Authorization bill for the first time in 48 years. And even if it means our troops do not get the funding and the protections they need,” he added. “It’s quite clear they are trying to run out the clock and just don’t want to get this bill done.”