WASHINGTON – In a last-ditch effort to avert a collision over judicial nominees, a bipartisan group of senators distanced themselves from Democratic and Republican leaders Thursday to try to strike a compromise certain to anger many colleagues.

As the negotiators talked privately into the night for a fourth consecutive day, their colleagues filled the chamber with speeches praising and denouncing the GOP leadership’s proposal to ban filibusters of judicial nominees. Majority Leader Bill Frist, R-Tenn., said he would file a motion Friday to end debate, anticipating a showdown vote Tuesday.

Even though Republicans hold 55 of the Senate’s 100 seats, lawmakers in both parties say it is unclear whether Frist can secure the 51 votes needed to trigger what many call the “nuclear option.” Frist said debate would continue for “as many hours as it takes for senators to air their views. But at some point, that debate should end and there should be a vote.”

The negotiators – about a half dozen from each party – said it was increasingly clear that party leaders would not accept the types of concessions needed to forge a deal that would derail Tuesday’s vote. But if the 12 could make a pact, they would represent the crucial votes needed to prevent further judicial filibusters this year without wiping out the right to filibuster in future congresses.

“They tried to work it out in their own way,” Sen. Mark Pryor, D-Ark., said of Frist and Minority Leader Harry Reid, D-Nev. Members of both parties had hoped the leaders could resolve the long-simmering impasse, said Pryor, a key negotiator, “but it didn’t work out that way. Now we’re looking at a group of senators rather than our two leaders.”

Party leadership and loyalty are integral to the modern-day Senate’s organization and operations, but in this case, a rebel breakthrough may be the only way to avert a vote on the rule change. Neither Senate leader has much room to maneuver – Frist because he has insisted on up-or-down votes on all of President Bush’s nominees, and Reid because he has said the threatened rule change must be taken off the table. With neither man likely to budge, a compromise has to be imposed by freelancing outsiders, senators and aides say.

That’s what the “gang of 12” is trying to do, said Sen. Susan Collins, R-Maine. “The two leaders have worked really hard” to resolve the issue, she said, “but they’ve not been able to do so.” She added that “I remain optimistic that we will reach a compromise,” even though “it’s very difficult” because of the leaders’ entrenched positions and pressure from interest groups on the left and right.

Another negotiator, Sen. Joseph Lieberman, D-Conn., said a breakthrough deal is possible, at least in theory, because the Senate is so narrowly divided on the two questions driving the debate: Democratic-led filibusters that have blocked 10 of Bush’s appellate court nominees, and Frist’s threat to ban such tactics through a change in Senate rules. “It’s certainly mathematically doable , and it’s legislatively doable, too, but we’ve got more [work] to do,” Lieberman said during a midday break in negotiations in the office of Sen. John McCain, R-Ariz.

Negotiators said the toughest task is building sufficient mutual trust among the dozen so they feel confident that neither side will renege or abuse an understanding that cannot be written in air-tight legalisms. At least six Democrats would agree to filibuster no more judicial nominees this year – including any for the Supreme Court – except in “extraordinary circumstances,” the participants say. In return, at least six Republicans would pledge to oppose Frist’s effort to change the filibuster rule unless the Democrats broke their promise.

A top GOP staffer, speaking on background because of the talks’ sensitive status, said Frist and Reid “have spies in there,” meaning both men are kept apprised of key points by loyal senators. A near-breakthrough Wednesday night was squelched, the aide said, when Reid pressed Democratic participants to reject language he felt would make it easier for Republicans to break a pledge to oppose Frist’s rule change.

Congressional scholar Norman Ornstein of the American Enterprise Institute said the negotiations are difficult “because it requires a leap of faith on the part of senators, because there are no guarantees here.” A determination of “extraordinary circumstances” that would void the deal, he said, “would be in the eye of the beholder.”

On the floor, senators continued to debate the nomination of Priscilla R. Owen of Texas to the U.S. Court of Appeals for the 5th Circuit. The true issue, all agreed, is whether to bar judicial filibusters altogether, which would give Bush more leeway in naming a staunch conservative to the Supreme Court.

Sen. Edward Kennedy, D-Mass., said, “if Republicans persist in the course they have set, they will destroy the ‘compact of comity’ that enables the Senate to fulfill its constitutional responsibilities.”

Such comity already appeared scarce late in the day, when Sen. Rick Santorum, R-Pa., said Democrats have violated a long tradition of letting judicial nominees reach up-or-down confirmation votes.

He then denounced “the audacity of some [Democratic] members to stand up and say, ‘How dare you break this rule?’ It’s the equivalent of Adolf Hitler in 1942 saying, ‘I’m in Paris. How dare you invade me?”‘ Several liberal groups called the remarks out of bounds.

Staff writer Shailagh Murray contributed to this report.