With an upcoming Senate vote on President Donald Trump's Supreme Court nominee Neil Gorsuch, Democrats are preparing to play one of the only cards they have as the minority party in the legislature: a filibuster. 

One of the most commonly used parliamentary tactics in Congress, a filibuster is an indefinite delay in voting on a nominee or bill. It can only be ended by a cloture vote, and for a Supreme Court nominee that requires 60 votes. Right now, all 52 Republicans are committed to breaking any Democratic filibuster, meaning if 8 Democrats vote to cross the aisle, the federal appellate judge would likely be confirmed in an up or down vote. 

Filibustering a Supreme Court nominee for a vacant seat has never happened in U.S. history. The only time a Supreme Court justice has ever been filibustered in any way was in 1968, when President Lyndon Johnson was prevented from elevating Associate Justice Abe Fortas to the Chief Justice position.

Democrats currently have 40 of the 41 votes they need to filibuster, with 48 Democrats in the Senate total. Three senators, Joe Donnelly (Ind.), Heidi Heitkamp (N.D.), and Joe Manchin (W.Va.) have announced they'll vote with Republicans to break the filibuster. The seemingly shocking display of disloyalty actually depicts a reality: All three senators are up for re-election in 2018, and represent deeply Republican states that voted for Trump in 2016.

Three other senators facing re-election in states that Trump won by five points or more have committed to join the filibuster: Sherrod Brown (Ohio), Claire McCaskill (Mo.), and Jon Tester (Mont.)

This leaves five Democratic senators who are publicly undecided on the filibuster, of which Democrats will need three to block Gorsuch: Michael Bennet (Colo.), Ben Cardin (Md.), Chris Coons (Del.), Angus King (Maine), and Robert Menendez (N.J.). Three previously undecided Democrats, California's Dianne Feinstein, Vermont's Patrick Leahy and Virginia's Mark Warner, announced on Monday that they would join the filibuster. 

There's been little indication as to how the remaining undecided Democrats will vote, though according to New York Magazine "Cardin [has] said [he’ll] ultimately vote against Gorsuch, but suggested [he] may join with Republicans on the procedural vote because [he's] against filibustering Supreme Court nominees. Coons expressed doubts about Gorsuch and his ability to overcome the filibuster, but [hasn't] explained where [he stands]. Bennet, Menendez, and King have offered few clues about how they’ll vote."

The anti-Trump resistance has mobilized a phone campaign to call undecided Senators to urge them to vote against cloture, and Democrats are confident they'll get the remaining votes, with Senate Minority Leader Chuck Schumer of New York declaring on Meet the Press that "It’s highly, highly unlikely that he’ll get to 60."

But Republicans fought for a year to keep Scalia's seat open, and have options as well. They can withdrawal the nomination, but far more likely would be the nuclear option—changing the rules remove the 60-vote threshold for a filibuster of a Supreme Court nominee. Senate Democrats used the same option in 2013, voting to remove the 60 vote cloture requirement for political and judicial nominees due to a backup of Republican filibusters on Obama administration picks. 

A number Republicans have expressed reluctance to do this, including Majority Leader Mitch McConnell. However, unless every remaining undecided Democrat votes yes on cloture, the nuclear option would be the only way Republicans can avoid a humiliating defeat that would scuttle a year's worth of obstruction, and deeply embarrass the embattled Trump administration. 

Gorsuch has been praised for his qualifications, but has also generated controversy among Democrats for his opinion in the Hobby Lobby religious freedom case, and for rulings seen as favoring corporations and law enforcement.