A bipartisan coalition of Senators believe it is close to a deal that would avert the looming showdown between Republicans and Democrats over judicial filibusters.

The potential deal, spearheaded by Sens. Trent Lott (R-Miss.) and Ben Nelson (D-Neb.), would involve at least a half-dozen Senators from each party signing a letter or memorandum of understanding that signals how they would proceed to vote on all matters related to judicial nominations.

The six Senate Republicans would commit to opposing the so-called nuclear option to end judicial filibusters, which would leave GOP leaders short of the 50 votes they need to execute the parliamentary move to abolish the procedure.

In exchange, the six Senate Democrats would pledge to allow votes on four of the seven circuit court nominees who were already filibustered in the 108th Congress and have been renominated.

Perhaps more importantly, the six Democrats would pledge to vote for cloture to end filibuster attempts on all other judicial nominees named by President Bush, including Supreme Court picks, except in “extreme circumstances,” according to a senior aide familiar with the discussions.

The aide familiar with the discussions declined to say which four circuit court nominees would be approved.

“It’s very close,” the aide said of the deal, requesting anonymity.

The precarious deal — which would last at least through the 2006 elections — still hasn’t been finalized, and neither side would reveal who was a party to it.

The agreement would not require a single vote to be cast for it to be executed. As long as each side has at least six Senators willing to uphold it, Senate Republicans would be unable to carry out the nuclear option and Senate Democrats would be unable to execute a successful filibuster.

If Frist wanted to, he could still force a vote on the parliamentary move to end the filibuster — something he is widely expected to do sometime this month — but it would face certain defeat if the six Republicans held strong.

Nelson and Lott were both traveling late last week and were not available to comment on the potential deal.

In remarks that went widely unnoticed in Washington, Sen. Mark Pryor (D-Ark.) told reporters in Little Rock on Thursday that he was working with Lott and Nelson on the deal. “It looks like there are a lot of Senators who want to avoid the nuclear option, and those are Senators on both the Republican and Democrat sides,” Pryor told reporters attending the news conference.

The deal hinges on whether Republicans decide that they can trust the half-dozen Democrats to invoke cloture on all other nominees, and on how the Democrats define “extreme circumstances.”

Senate Minority Leader Harry Reid (D-Nev.) made a somewhat similar compromise offer to Majority Leader Bill Frist (R-Tenn.) two weeks ago that involved some number of circuit court nominees getting approved and others getting jettisoned. An unofficial part of that offer was that Reid would ensure that filibusters would only occur under “extraordinary circumstances” — a phrase he even uttered in a floor speech.

Republicans rejected that proposal as overly vague, and some, including Sen. John Ensign (R-Nev.), noted it was difficult to assess such an offer that was not in writing.

The Lott-Nelson deal, however, would be in writing, and if Republicans eventually decided that the six Democratic signatories went back on their word by supporting unnecessary filibusters, they could presumably back out of the deal and tell Frist they were ready to support the nuclear option.

In the Lott-Nelson arrangement, Senators have been talking one-on-one, and there only needs to be trust among those dozen or more supporters of the effort to execute the deal.

Such a deal would likely be seen as a major setback for both interest groups on the right and left, who have been ratcheting up the pressure on the half dozen or so GOP Senators who have not yet announced their position on Frist’s effort to change the rules.

Conservative groups made clear, after the Reid proposal, that any deal not guaranteeing that all previously filibustered nominees receive confirmation would be viewed as insufficient. For their part, liberal groups have issued proclamations that the right to filibuster future nominees — including Supreme Court picks — is essential.

One senior liberal activist said last week that the only way to avert a nuclear showdown would be in a deal crafted by the undecided Republicans, noting that few expect the tense relationship between Frist and Reid to cool in time for those two to craft a deal.

The aide familiar with the Lott-Nelson talks specifically declined to name the other Senators involved, for fear that groups on the right and left would bombard their offices with calls, faxes and e-mails in an effort to scuttle the potential deal.

Despite the negotiations, leadership aides on both sides continued to prepare for the showdown, with Republicans and Democrats planning a major communications onslaught this week.

Sen. John Cornyn (R-Texas) plans to mark today’s four-year anniversary of the day Bush announced his first batch of judicial nominees, including still-embattled Texas Supreme Court Justice Priscilla Owen, at a press event with Latino leaders from Texas. He’ll also take to the floor with a strongly worded speech attacking Democratic filibusters.

In the meantime, a conservative group, the National Coalition to End Filibusters, will host a press conference today at the National Press Club with a series of anti-filibuster speakers, including Hiram Lewis, a 2006 GOP Senate candidate against Sen. Robert Byrd (D-W.Va.). Byrd, who has delivered a series of floor attacks against the nuclear option, has become a favorite target of conservatives.

But Democrats and their allies say they’ll fight back just as strongly, continuing to maintain an edge in television advertising in key states and mounting a series of events this week.

In a sign of how personal the battle has become, Democrats plan to bring to the Capitol this week a group of Princeton University students who have been holding a filibuster-style protest outside the Frist Campus Center, a building endowed by the Senate Majority Leader’s family. The Senator is a graduate of Princeton, where his son Harrison currently attends, and he has continued to be a large donor to the school.

In addition, liberal activists planned to protest a speech Sunday by Sen. George Allen (R-Va.) at Regent University, which was founded by televangelist Pat Robertson. Robertson is the focus of ads run by MoveOn.org’s political committee for remarks he made saying federal judges are a bigger threat to the nation than the terrorists of Sept. 11, 2001.

Democratic aides monitoring the situation say that MoveOn ads are still running in Virginia, Maine, Oregon and Nebraska, targeting five swing GOP votes: Sens. John Warner (Va.), Olympia Snowe (Maine), Susan Collins (Maine), Gordon Smith (Oregon) and Chuck Hagel (Neb.).