What will you do when terrorists attack, or U.S. friction with some foreign power turns into a military confrontation? I don’t mean in your personal life, where you should keep calm and carry on. I mean politically. Think about it carefully: The fate of the republic may depend on your answer.

Of course, nobody knows whether there will be a shocking, 9/11-type event, or what form it might take. But surely there’s a pretty good chance that sometime over the next few years something nasty will happen — a terrorist attack on a public place, an exchange of fire in the South China Sea, something. Then what?

After 9/11, the overwhelming public response was to rally around the commander in chief. Doubts about the legitimacy of a president who lost the popular vote and was installed by a bare majority on the Supreme Court were swept aside. Unquestioning support for the man in the White House was, many Americans believed, what patriotism demanded.

The truth was that even then the urge toward national unity was one-sided, with Republican exploitation of the atrocity for political gain beginning almost immediately. But people didn’t want to hear about it; I got angry mail, not just from Republicans but from Democrats, whenever I pointed out what was going on.

Unfortunately, the suspension of critical thinking ended as such suspensions usually do — badly. The Bush administration exploited the post-9/11 rush of patriotism to take America into an unrelated war, then used the initial illusion of success in that war to ram through huge tax cuts for the wealthy.

Bad as that was, however, the consequences if Donald Trump finds himself similarly empowered will be incomparably worse.

We’re only three weeks into the Trump administration, but it’s already clear that any hopes that Mr. Trump and those around him would be even slightly ennobled by the responsibilities of office were foolish. Every day brings further evidence that this is a man who completely conflates the national interest with his personal self-interest, and who has surrounded himself with people who see it the same way. And each day also brings further evidence of his lack of respect for democratic values.

You might be tempted to say that the latest flare-up, over Nordstrom’s decision to drop Ivanka Trump’s clothing line, is trivial. But it isn’t. For one thing, until now it would have been inconceivable that a sitting president would attack a private company for decisions that hurt his family’s business interests.

But what’s even worse is the way Sean Spicer, Mr. Trump’s spokesman, framed the issue: Nordstrom’s business decision was a “direct attack” on the president’s policies. L’état, c’est moi.

Mr. Trump’s attack on Judge James Robart, who put a stay on his immigration ban, was equally unprecedented. Previous presidents, including Barack Obama, have disagreed with and complained about judicial rulings. But that’s very different from attacking the very right of a judge — or, as the man who controls 4,000 nuclear weapons put it, a “so-called judge” — to rule against the president.

The really striking thing about Mr. Trump’s Twitter tirade, however, was his palpable eagerness to see an attack on America, which would show everyone the folly of constraining his power:

Just cannot believe a judge would put our country in such peril. If something happens blame him and court system. People pouring in. Bad!

Never mind the utter falsity of the claim that bad people are “pouring in,” or for that matter of the whole premise behind the ban. What we see here is the most powerful man in the world blatantly telegraphing his intention to use national misfortune to grab even more power. And the question becomes, who will stop him?

Don’t talk about institutions, and the checks and balances they create. Institutions are only as good as the people who serve them. Authoritarianism, American-style, can be averted only if people have the courage to stand against it. So who are these people?

It certainly won’t be Mr. Trump’s inner circle. It won’t be Jeff Sessions, his new attorney general, with his long history of contempt for voting rights. It might be the courts — but Mr. Trump is doing all he can to delegitimize judicial oversight in advance.

What about Congress? Well, its members like to give patriotic speeches. And maybe, just maybe, there are enough Republican senators who really do care about America’s fundamental values to cross party lines in their defense. But given what we’ve seen so far, that’s just hopeful speculation.

In the end, I fear, it’s going to rest on the people — on whether enough Americans are willing to take a public stand. We can’t handle another post-9/11-style suspension of doubt about the man in charge; if that happens, America as we know it will soon be gone.

Paul Krugman

Paul Krugman is an American economist and journalist who received the 2008 Nobel Prize for Economics for his work in economic geography and in identifying international trade patterns. He was also known for his op-ed column in The New York Times.