Democracy depends on norms as well as law, and respecting established norms is essential in a diverse society. The norms that get layered on top of laws are what enable groups with fundamentally different ideas and objectives to live and work together. And if the past decade has taught us anything, it is that a politics of abandoning norms to win today’s battle is mutilating our democracy.

So, yes, Republicans had the legal power to refuse a hearing to Judge Merrick Garland even though he was nominated nearly eight months before the 2016 election, just as they had the legal power to ram Amy Coney Barrett’s nomination through the Senate Judiciary Committee two weeks before the Nov. 3 election.

And yes, they had the legal power to do so even while offering disgracefully hypocritical justifications: denying Judge Garland a hearing because, they said, legitimacy required waiting for an election that was close in time, while rushing through a last-minute appointment for Judge Barrett lest they lose an election that’s much, much closer. But both acts betrayed a ruthless willingness to politicize judicial selection in extreme ways that upended long-established norms.

Liberals say that if Joe Biden wins the election, Democrats should answer by adding justices to the Supreme Court. Republicans respond with faux outrage that this would politicize the judiciary. But they have already politicized the judiciary. The question is whether only one side should play that game. Besides, not only is enlarging the Supreme Court legal, its size has changed seven times over its history.

Adding judges would be a political response to a political act. But the extremes to which Republicans have been willing to go leave the Democrats no other choice. Not for revenge or because turnabout is fair play, but as the only way back to a less politicized process.

This is a lesson we learned decades ago from economists and game theorists: Once cooperation breaks down, the only play to restore it is tit-for-tat. It’s the only way both sides can learn that neither side wins unless they cooperate.