By Paul Krugman

Once upon a time, the death of a Supreme Court justice wouldn’t have brought America to the edge of constitutional crisis. But that was a different country, with a very different Republican Party. In today’s America, with today’s G.O.P., the passing of Antonin Scalia has opened the doors to chaos.

In principle, losing a justice should cause at most a mild disturbance in the national scene. After all, the court is supposed to be above politics. So when a vacancy appears, the president should simply nominate, and the Senate approve, someone highly qualified and respected by all. In reality, of course, things were never that pure. Justices have always had known political leanings, and the process of nomination and approval has often been contentious. Still, there was nothing like the situation we face now, in which Republicans have more or less unanimously declared that President Obama has no right even to nominate a replacement for Mr. Scalia and no, the fact that Mr. Obama will leave soon doesn’t make it O.K. (Justice Kennedy was appointed during Ronald Reagan’s last year in office.)

Nor were the consequences of a court vacancy as troubling in the past as they are now. As everyone is pointing out, without Mr. Scalia the justices are evenly divided between Republican and Democratic appointees which probably means a hung court on many issues. And there’s no telling how long that situation may last. If a Democrat wins the White House but the G.O.P. holds the Senate, when if ever do you think Republicans would be willing to confirm anyone the new president nominates?

How did we get into this mess?

At one level the answer is the ever-widening partisan divide. Polarization has measurably increased in every aspect of American politics, from congressional voting to public opinion, with an especially dramatic rise in“negative partisanship” distrust of and disdain for the other side. And the Supreme Court is no different. As recently as the 1970s the court had several “swing” members, whose votes weren’t always predictable from partisan positions, but that center now consists only of Mr. Kennedy, and only some of the time. But simply pointing to rising partisanship as the source of our crisis, while not exactly wrong, can be deeply misleading. First, decrying partisanship can make it seem as if we’re just talking about bad manners, when we’re really looking at huge differences on substance. Second, it’s really important not to engage in false symmetry: only one of our two major political parties has gone off the deep end.

On the substantive divide between the parties: I still encounter people on the left (although never on the right) who claim that there’s no big difference between Republicans and Democrats, or at any rate “establishment” Democrats. But that’s nonsense. Even if you’re disappointed in what President Obama accomplished, he substantially raised taxes on the rich and dramatically expanded the social safety net; significantly tightened financial regulation; encouraged and oversaw a surge in renewable energy; moved forward on diplomacy with Iran.

Any Republican would undo all of that, and move sharply in the opposite direction. If anything, the consensus among the presidential candidates seems to be that George W. Bush didn’t cut taxes on the rich nearly enough, and should have made more use of torture. When we talk about partisanship, then, we’re not talking about arbitrary teams, we’re talking about a deep divide on values and policy. How can anyone not be “partisan” in the sense of preferring one of these visions? And it’s up to you to decide which version you prefer. So why do I say that only one party has gone off the deep end? One answer is, compare last week’s Democratic debate with Saturday’s Republican debate. Need I say more?

Beyond that, there are huge differences in tactics and attitudes. Democrats never tried to extort concessions by threatening to cut off U.S. borrowing and create a financial crisis; Republicans did. Democrats don’t routinely deny the legitimacy of presidents from the other party; Republicans did it to both Bill Clinton and Mr. Obama. The G.O.P.’s new Supreme Court blockade is, fundamentally, in a direct line of descent from the days when Republicans used to call Mr. Clinton “your president.” So how does this get resolved? One answer could be a Republican sweep although you have to ask, did the men on that stage Saturday convey the impression of a party that’s ready to govern? Or maybe you believe based on no evidence I’m aware of that a populist rising from the left is ready to happen any day now. But if divided government persists, it’s really hard to see how we avoid growing chaos. Maybe we should all start wearing baseball caps that say, “Make America governable again.”