With complications mounting for Republican efforts to rally 60 votes to confirm Neil Gorsuch to the Supreme Court, you’re going to hear a lot in the next few weeks about the so-called “nuclear option.”

Here’s what it all means:

Let’s start with the mathematics.

There are 52 Republicans in the Senate and 48 senators who caucus with the Democrats.

It only takes a simple majority to confirm a Supreme Court nominee. But it takes 60 votes to break a filibuster on a Supreme Court nominee. Never before has there been a successful filibuster of a Supreme Court nominee.

The Senate filibustered the nomination of Associate Justice Abe Fortas to become chief justice of the United States in 1968. But Fortas was already on the Supreme Court. Nominees have withdrawn. President George W. Bush nominated Harriet Miers in 2005 before she pulled out, anticipating a rocky confirmation process. President Ronald Reagan tapped Douglas Ginsburg for the Court in 1987. But Ginsburg withdrew over his marijuana use. Twelve Supreme Court nominees went down to defeat outright on the Senate floor. The most recent Supreme Court nominee to stumble at confirmation came in 1987 when Reagan selected Robert Bork. Bork only garnered 42 ayes for confirmation and the nomination failed.

Senate Democrats have made it clear that they intend to filibuster Gorsuch and make Senate Republicans cough up 60 yeas to break their filibuster.

Senate Majority Leader Mitch McConnell, R-Ky., insists that the Senate will fact confirm Gorsuch.

But how do they get there?

Republicans think they can persuade some Democratic senators who represent swing states who face re-election in 2018 to at least vote to break the filibuster (known as voting for cloture) if not voting to confirm Gorsuch. Those senators include Sens. Bill Nelson, D-Fla.; Joe Donnelly, D-Ind.; Angus King, I-Maine; Debbie Stabenow, D-Mich.; Claire McCaskill, D-Mo.; Jon Tester, D-Mont.; Heidi Heitkamp, D-N.D.; Sherrod Brown, D-Ohio; Bob Casey, D-Pa.; Tim Kaine, D-Va.; Joe Manchin, D-W.Va.; and Tammy Baldwin, D-Wis.

But if not, Republicans are stuck. They can’t get Gorsuch to a final vote.

They say paybacks are hell. And both sides have a lot to say about paybacks right now.

For Democrats, this is about Republicans failing to ever hold a confirmation hearing for President Obama’s Supreme Court nominee, Merrick Garland. For Republicans, this is about Democrats taking the extraordinary step in 2013 to lower the bar to from 60 to 51 to break filibusters for all Executive Branch nominees except Supreme Court nominees.

This was known as the “nuclear option” and cast the Senate into “nuclear winter” for more than a year.

Both sides feel the other side wronged them. Now, this is about revenge.

Let’s go back to the fall of 2013 when Democrats teed up the nuclear option.

Democrats intensified their criticisms of Republicans when the GOP stalled the nomination of Patricia Millett to serve on the U.S. Court of Appeals for the District of Columbia. The Senate didn’t clear a procedural hurdle which required 60 votes to break a GOP filibuster of the Millett nomination.

The filibuster is the quintessence of the Senate. The right of the minority to stall and delay – even upend the agenda the majority party intends to advance. Stripping the Senate of at least part of that opportunity for dissent fundamentally changes the place. But these filibusters stymied general Senate business and drove Democrats up a wall.

So, they hit the nuclear button.

It should be noted that the “nuclear option” is not a rules change, but a change in precedent. But much of what the Senate does is based on precedent.

The Senate must be in a very unique parliamentary posture in order to detonate a “nuclear option.” It has to get onto a parliamentary item which is “non-debatable.” In other words, senators can’t demand more time to speak on a given topic, and a senator cannot appeal how the chair rules on a parliamentary question. That is the key. If a senator can challenge how the chair has ruled, they can gum up the works. But if the chair establishes a new precedent by ruling something to be in or out of order, no one can contest that ruling.

And therefore, the Senate establishes a new precedent.

This is how it worked in 2013: That year, then-Senate Majority Leader Harry Reid, D-Nev., tried to proceed to consider again a failed vote to end debate on the Millett nomination for the D.C. Circuit. That motion was non-debatable … so the GOP couldn’t stop him. So, the Senate voted to make the failed vote the business at hand before the Senate.

At that point, Reid made a point of order that the votes required to break a filibuster on “all nominations other than for the Supreme Court of the United State States is by majority vote.” The presiding officer (a Democrat) ruled against Reid (this was pre-baked). After all, that was the old precedent. So Reid then appealed the chair’s ruling. That forced the Senate to vote on what the chair ruled: a simple majority wasn’t valid to end filibusters on Executive Branch nominees. Just a supermajority (60). But on the appeal, the Senate voted against the chair. That affirmed Reid’s position. The Senate overrode the chair’s ruling, establishing a new precedent to break filibusters on all nominations except the Supreme Court.

The Senate then voted to end the filibuster on the Millett nomination with just 55 yeas (not 60, as per the old way). The Senate then confirmed Millett, 56-38.

And thus, the Senate established a new precedent.

For a new nuclear option, McConnell would have to pitch the Senate into a special parliamentary posture in order to mimic Reid’s 2013 gambit and lower the bar to break a possible filibuster against Gorsuch.

The question is whether he has 51 senators (or 50 senators and Vice President Pence) willing to go along with the tactic. Keep in mind that Republicans were very sad that Reid hit the nuclear switch in 2013. They felt it diminished the Senate and its history of “unlimited debate.” Republicans and some Democrats didn’t like that the Senate was changing the bar for filibusters, because, well, they may like to filibuster a given issue sometime.

It is unclear if McConnell, an institutionalist, wishes to go that route. It is risky. And he may not even have the votes to get Republican senators to vote against the ruling of the chair to establish a new precedent to end filibusters on Supreme Court nominees.

There are also questions as to whether Senate Minority Leader Chuck Schumer, D-N.Y., may want to truly go to the mat on this nominee or put vulnerable Democratic senators up for re-election next year in challenging states on the hook. Confirming a conservative to take the place of a conservative like Antonin Scalia on the court may be okay for some Democrats. It doesn’t disrupt the balance of the court. But confirming another conservative to take the place of say, a liberal, like Ruth Bader Ginsburg, is entirely different.

There is another way for Gorsuch to wind up on the Supreme Court if he can’t overcome a filibuster: a recess appointment.

Article I, Section 5 of the Constitution says that “Neither House, during the Session of Congress, shall, without the Consent of the other, adjourn for more than three days, nor to any other Place than that in which the two Houses shall be sitting.” In other words, the House and Senate must earn the blessing of the other before taking off for more than three days. If they don’t, the House and Senate sometimes meet for just a few seconds at a time at three-day intervals with skeleton staff. These are called “pro-forma” sessions and constitute a “meeting” of the House or Senate.

The founders anticipated periodic congressional recesses. So to maintain the operation of government, the founders grafted a clause onto Article II, Section 2 of the Constitution. It states that “The President shall have Power to fill up all Vacancies that may happen during the Recess of the Senate.”

In 2012, President Obama made four recess appointments during a three-day respite between two pro-forma sessions. Opponents of the effort challenged the president’s appointments to the Supreme Court. Justices ruled Obama’s ploy to be unconstitutional. In National Labor Relations Board v. Canning, Supreme Court Justice Stephen Breyer wrote that it was out of bounds for the Executive Branch to tell the Legislative Branch what constitutes a recess.

“The Senate is in session when it says it is,” wrote Breyer in the majority opinion.

Moreover, the court also determined that a recess between three and 10 days is too abbreviated for a recess appointment. In other words, the Senate has to be gone for more than 10 days to make the recess appointment valid.

So, the Republican House and Senate would have to work together in order to engineer a recess, thus opening the door to a recess and giving Trump the window he needs for a possible recess appointment.

Recess appointments to the Supreme Court aren’t unprecedented. President Dwight Eisenhower advanced two individuals to the high court via recess appointments: William Brennan in 1956 and Potter Stewart in 1958. However, all recess appointments are temporary. The Senate later confirmed both Brennan and Stewart.