More than a dozen Senate centrists entered tense negotiations Tuesday evening over the details of a proposed compromise on judicial filibusters, convening a series of bipartisan, closed-door meetings to hammer out final language and ease concerns on all sides.

The group, now working under the auspices of the revamped Centrist Coalition, says that enough Democratic and Republican Senators are considering signing the memo that heading off the imposition of the so-called nuclear option is now a real possibility.

The group held three separate meetings Tuesday afternoon and evening, breaking from Sen. John Warner’s (R-Va.) office shortly before 7 p.m. without an agreement, according to one aide. The group expects to meet again today.

Broadly, signatories of the centrist-sponsored memo would commit to opposing both the nuclear option and future filibusters, while allowing more than half of the already filibustered judicial nominees to be approved.

But the final wording of the document — and deciding which of the seven filibustered nominees would be cleared for confirmation — was proving to be a major sticking point.

“If there’s a way to come to closure on this tonight, we will do it,” said Sen. Ben Nelson (D-Neb.), a leader of the effort along with Sens. John McCain (R-Ariz.), Mark Pryor (D-Ark.) and Ken Salazar (D-Colo.).

Giving the proposal more heft and credibility, each side has lined up the potential support of Senators who had until recently been considered in the camps of irreconcilables on the judicial issue.

Sen. Lindsey Graham (R-S.C.), who in 2003 seriously researched filing a lawsuit against Senate Democrats to end judicial filibusters, is now part of the group working to sign on to a deal even though it will likely leave three nominees stymied by filibuster.

And Sen. Robert Byrd (D-W.Va.), whose speeches in defense of the filibuster earlier this spring became a rallying cry to liberal activists, joined a meeting of about a dozen Senators in Warner’s office Tuesday afternoon to review various proposals for the memorandum of understanding, according to aides.

Byrd said earlier in the day that he liked the general outline of the proposal being put forward by Nelson and others, adding that it would end the chance of a the nuclear option and leave the potential for a filibuster intact.

“I’m for his approach,” he said as he entered his party’s weekly luncheon. “Let’s avoid this hellish nuclear option. Let’s keep freedom of speech.”

Sen. Olympia Snowe (R-Maine), who has voiced concerns about the party-line evisceration of judicial filibusters, said she thought there was momentum toward a deal. “I would say more likely than not,” she said before the critical afternoon sessions.

After a tense 70-minute meeting that included both the Centrist Coalition, Majority Leader Bill Frist (R-Tenn.) and Minority Leader Harry Reid (D-Nev.), the coalition broke apart into a series of smaller meetings.

Graham and Sen. John Warner (R-Va.), for example, walked out of the second floor Capitol suite and headed together toward the Russell Senate Office Building, where the centrist group eventually reconvened early Tuesday evening.

The basic structure of the deal remains intact as it was first reported by Roll Call on May 9.

Variously worded draft memoranda were believed to be circulating among Senators, but each contained a list of four basic principles:

• That four of the seven filibustered nominees would be approved for a vote, with California Supreme Court Justice Janice Rogers Brown and two of those from the 6th U.S. Circuit Court of Appeals potentially on the approval list;

• That three would be rejected, with William Myers to the 9th Circuit and Henry Saad to the 6th Circuit possibly on the filibustered list;

• That at least six Democrats would foreswear any other filibusters except in “extraordinary circumstances”;

• That at least six Republicans would foreswear Frist’s effort to end filibusters by the parliamentary, party-line vote now known commonly as the nuclear option.

Some aides suggested that one of the major stumbling blocks was determining the fates of Texas Supreme Court Justice Priscilla Owen and 11th Circuit Judge William Pryor. Some members of the group are pushing to include Owen as the final judge to be confirmed, while others, including Graham, are pushing for Pryor, said an aide with knowledge of the talks.

But at least two other aides suggested that the biggest sticking point would be the wording of the third and fourth points in the memo.

One draft memo obtained by Roll Call said that even if any of the signatories to the deal voted to sustain a filibuster of another nominee, that would not allow anyone to back out of the deal and then support the nuclear option.

These aides suggested that Republicans were insisting on a wording that would give them an out clause if Democratic signatories acted in bad faith.

Nelson said that the wording of points three and four were a hot topic in that meeting. He suggested that Senators will ultimately have to take a leap of faith with each other.

“That’s the question. My comment was, it’s about trust,” he said.

With both Democratic and Republican leadership still girding for the battle — debate on Owen or Rogers Brown is expected to officially begin this morning — the centrists wanted to nail down their pact soon but refused to be pinned down by a timetable.

“People realize we’re reaching the pivotal moment,” Snowe said.

On the Democratic side, Nelson, Pryor and Salazar have been leading the effort to round up support. In addition to Byrd, Sens. Joe Lieberman (D-Conn.) and Mary Landrieu (D-La.) have also participated in the talks, several aides said.

Also attending the meeting with the Centrist Coalition and the leaders were Sens. Tom Carper (D-Del.), Blanche Lincoln (D-Ark.) and Bill Nelson (D-Fla.).

On the Republican side, McCain has led the charge and is expected to have the firm support of Snowe and Sen. Susan Collins (R-Maine), with Warner and Graham very close to supporting a compromise if the wording on the deal meets their standards, sources say.

In addition to Graham, the coalition meeting also drew two other members of the Judiciary Committee: Chairman Arlen Specter (R-Pa.) and Sen. Mike DeWine (R-Ohio).

The deal would not be supported by Frist, who has lobbied in favor of the rule change.

If all those in attendance at the meetings were to sign on to a deal, it could raise a serious question of how much party discipline Republicans can enforce.

Of those considering supporting a deal to avert the showdown, at least five are committee chairmen: Warner (Armed Services), Specter (Judiciary), McCain (Indian Affairs), Collins (Homeland Security and Governmental Affairs) and Snowe (Small Business).

But the wavering Democrats are also likely to face fierce criticism from their normally strong allies on the left. Informed of the proposal’s basic structure, Nan Aron, president of the Alliance for Justice, was flabbergasted that Democrats would consider approving Rogers Brown for confirmation along with either Pryor or Owen.

Judges, Aron said, “shouldn’t be horse-traded for the sake of political expediency.