Barring a last-minute intervention, Senate Republicans are poised to go nuclear Thursday. They say they will reinterpret the rules to empower a simple majority to cut off debate and confirm Neil Gorsuch, President Trump’s Supreme Court nominee.

Partisans disagree about who is to blame for bringing the Senate to the nuclear brink. Republicans fault Democrats for stoking the war by filibustering George W. Bush’s judicial nominees and going nuclear in 2013 over executive and lower court appointees. Democrats blame Republicans for their reflexive opposition to President Obama’s nominees, including the GOP’s unprecedented refusal to consider Obama’s Supreme Court nominee, Merrick Garland.

Arbitrating the blame game is difficult, not least because where politicians stand depends on where they sit — and they periodically change seats. So instead, let’s consider this week’s potential nuclear confrontation as the next step in the Senate’s decades-long, intensely partisan procedural warfare — a battle increasingly pitched over the judicial branch.

How did we get here and what might come next for the Senate?

Partisanship is boiling over

Since at least the 1980s, partisanship has been on a steady rise in the Senate — as various ideological positions (and the politicians who hold them) have been sorted into their respective parties and as elections have gotten ever more competitive.

We can measure the increasing partisanship in a couple of ways. First, consider the evidence of increasing partisan conflict in the outcomes of Senate cloture votes. As I’ve written here before, the Senate “cloture” rule requires 60 votes to cut off debate (or 67, for motions to debate changes to the rules). Once debate is ended by invoking “cloture,” 30 hours of post-cloture debate must elapse — unless all 100 senators agree to waive it. Only then does the Senate take a simple-majority vote on the measure or motion. After cloture, remaining amendments must be narrowly related to the underlying bill.

The figure below captures average party differences on cloture votes, which is to say, the difference between the percentage of majority and minority party senators voting yea. (If 90 percent of the majority party and just 20 percent of the minority party vote in favor of cloture, the difference is 70 percent.)

Remarkably, procedural partisanship has more than doubled since the 1970s — meaning that party lines are more sharply drawn today than in the past. You can’t see this in the figure, but since the first Bush administration, Democrats have filed for cloture significantly more often than have Republicans — suggesting that the GOP has more aggressively tried to block the majority than have Democrats.

Courts have become more important policymakers

Contested judicial nominations are another measure of just how polarized the nation has become. For instance, few people are surprised by the partisan furor about Trump’s nomination of Gorsuch (and before that, Obama’s nomination of Garland) to the Supreme Court. At a time when the partisan divide is so strong, ambitious majorities naturally want to tilt the courts to favor their own policy agendas. The tougher it is for Congress to legislate, the more power flows to the courts. That has especially been true in the Supreme Court in recent years. Legislative stalemate has made the Supreme Court pivotal to the fate of national issues that might otherwise have been settled by Congress or state legislatures, including health care, abortion rights, immigration, education and more.

Looking more narrowly at the fate of judicial nominees to the lower federal courts, rising levels of partisan disagreement over who should sit on the bench drive down confirmation rates — starting with Courts of Appeals nominees in the late 1980s and spilling over to the less controversial trial courts about a decade later.

Note in particular how confirmation becomes rocky for both appellate and district court appointees during the Bush and especially the Obama years. As partisanship heated up and courts became more salient to lawmakers, trial court nominees no longer escaped partisan crosshairs.

A long-running parliamentary arms race 

As Steve Smith argues, episodic obstruction by the minority — followed by retaliation by the majority — is part of a longer-term parliamentary arms race. Cohesive parties governed by the Senate’s byzantine rules fully expect the other party to exploit its parliamentary rights. That party in turn knows it will face the consequences of a retaliatory majority.

That’s what we saw when Republicans obstructed Obama’s executive and judicial appointees in 2012 and 2013 — and the Democrats responded by going nuclear (i.e., eliminating the filibuster) for those lower court nominees. As you can see in the figure above, creating majority cloture for nominations ensured confirmation of almost every Obama judicial nominee by the end of 2014. In response, when the GOP regained control of the Senate in 2015, Republicans nearly ground to a halt advice and consent for all federal courts — including Garland’s nomination to the Supreme Court.

Gorsuch’s nomination has accelerated this parliamentary arms race. As the Democrats threaten to filibuster, Republicans have promised to go nuclear.

It is tempting to view Democrats’ behavior as payback for Republican norm-breaking that blocked Garland. And in the short term, it is. But the longer view reveals an equally important dynamic. As the parties become ever more polarized, party majorities have been looking to the Senate rules to find new ways to pursue their agendas.

A march toward majority rule?

So what’s next? This week, Senate Majority Leader Mitch McConnell (R-Ky.) said the filibuster would remain in place for legislation. But how long will that be true?

Reinterpreting Senate rules by majority vote has long been technically possible. But the Democrats’ nuclear move in 2013 showed senators that limiting minority rights was politically feasible. We just saw the ease and speed with which Republicans pledged to eliminate Supreme Court filibusters, reinforcing the lesson of that 2013 change of rules. And it suggests that the legislative filibuster will eventually come under pressure.

Weighing against such a change is the institutional power that senators derive from the Senate’s lax rules of amendment and debate. For example, even majority party senators can take measures hostage with the threat of a filibuster, releasing their prey in exchange for action on a favored measure. Senators regardless of party also benefit from weak limits on amendments to focus attention on their policy or political priorities.

Keeping the legislative filibuster can also ironically serve the majority party’s interests. In this current Congress, requiring 60 votes to move forward on legislative measures might at times work to Republican Party advantage. Red-state Democrats, such as West Virginia’s Joe Manchin III, will at times be cross-pressured on votes — amplifying divisions among Democrats. The 60-vote requirement will also allow Republicans to blame Democrats for blocking parts of the Trump agenda, especially on measures that moderate Republicans might oppose. Republicans might also think about their future parliamentary needs and be wary of empowering the next Democratic majority with such powerful rules.

Still, the longer-term evolution of the Senate has followed a very slow path toward majority rule. Senators have rarely reversed course, suggesting that a cohesive and ambitious Senate party could one day be tempted to finish the march to majority rule.