Wednesday’s vote on whether to keep Caitlin Halligan off the nation’s second-most-important federal bench is a bellwether test about the future of the judicial wars.

Assuming the coming snow isn’t falling too hard, senators will decide at midday whether to advance Halligan past a GOP filibuster and toward one of the four vacant seats on the D.C. Circuit Court of Appeals – which, because of its jurisdiction over almost all cases where the federal government is a party, is generally considered the penultimate place of judicial power after the Supreme Court. (She would be the first Obama appointee to that court, which hasn’t welcomed a newcomer since 2006.) So far, though, Halligan can count on the support of all 55 Democrats but only one Republican: Lisa Murkowski, who says she’ll oppose confirmation but believes Halligan’s record is not so far outside the mainstream that she should be denied an up-or-down roll call.

Unless at least four others in the GOP agree to go at least that far – hewing to the same principal that allowed Chuck Hagel to become Defense secretary last week – the cloture vote will fail, Halligan’s three-year-old nomination will be dead, and the consequences will be undeniable: Several more solid precedents about the balance of power and the limits of partisanship in Washington will be out the window. Barack Obama would no longer be able to expect much if any latitude in shaping the federal bench to his demographic and ideological liking, as all his second-term predecessors have enjoyed. The plug that Republicans put in the judicial confirmation pipeline last year, citing past campaign season practices when the Senate’s not in the hands of the president’s party, looks to be kept in place long after Obama’s re-election. And the famous Gang of 14 accord, which defused a judicial confirmation battle royal  just when a parliamentary nuclear winter seemed inevitable, will have outlived its one-hailed-as-historic usefulness after less than a decade.

Advocates for avoiding all that are still holding out hope that the three Republicans from that “gang” who are still around – Lindsay Graham, John McCain and Susan Collins – will vote to at least advance Halligan to a final vote, and might be joined by one or more of the following: Orrin Hatch, Jeff Flake, Marco Rubio, Lamar Alexander, Mark Kirk or John Thune.

But Graham, McCain and Collins all voted to spike the Halligan nomination the first time it was on the floor, in December 2011. Their votes not only guaranteed the D.C. Circuit would have a majority of Republican-nominated judges through the election, but were also a clear signal the agreement they joined in 2005 was in serious jeopardy.

Back then, the Senate was brought to a standstill over the minority party’s use of the filibuster (the Democrats, at the time) to effectively create a 60-vote-majority requirement for appeals court judges nominated by a president from the opposite party (George W. Bush). The impasse was broken when seven senators from each party promised to oppose such tactics except in “extraordinary circumstances,” which were not further defined. In almost all circumstances, the 14 promised, they’d work to make sure nominations that made it as far as the floor got a simple-majority threshold confirmation vote.

On paper, Halligan looks unimpeachable: Six years as solicitor general of New York State, then three years as chief of the appellate practice at Weil, Gotshal & Manges, and the past three years as general counsel for the Manhattan D.A. But her Republican critics say her record, as both New York’s top appeals lawyer and in private practice, revealed her as a liberal activist. They are particularly galled with an argument, made on the state’s behalf, that firearms manufacturers should be held liable for crimes committed with the guns they made. They also criticized her legal positions on immigration, abortion and terrorism cases. (Behind the scenes, some are also ready to concede their main worry is about her shifting the ideological balance on the D.C. Circuit.)

Democrats counter that it would be an awful precedent to block a nominee based on positions taken at the behest of either government or corporate clients – and their favorite quote to buttress that view comes from none other than Chief Justice John Roberts. (Halligan, as it turns out, is up to fill the appeals seat that’s been vacant since Roberts’ promotion.) “It is a basic principle in our system that lawyers represent clients and you do not ascribe the position of the client to the lawyer,” Roberts told some Democratic skeptics in his 2005 confirmation hearings. It’s a position that goes back to John Adams and the Revolution.”

Like so much else in the capital, it’s another venerated precedent about regular order that’s now on the endangered list.