Mr. Trump nonetheless has many supporters. While a majority of Americans believes that the president is doing a bad job, around 40 percent of voters — and some 80 percent of Republicans — approve of his performance. A number of Republican senators and congressmen have reportedly objected to Mr. Trump’s attacks on Mr. Sessions and voted against parts of his legislative agenda, but most have yet to oppose him publicly.

This is worrying. The Constitution cannot defend itself. If Congress does not stand up to Mr. Trump because Republicans are afraid of their own base, the president may be able to obstruct the course of justice with impunity. Worse, he may then conclude that he can get away with violating even more basic limits on his power.

If Congress stands idly by as he fires Mr. Mueller, as it did when he fired the F.B.I. director, James Comey, it might prove similarly pliable should he disregard court rulings, attempt to close down critical newspapers or order his appointees at the Department of Justice to indict Mrs. Clinton.

Congress must send a clear message that these types of violations won’t be tolerated. If Mr. Trump fires Mr. Mueller, Congress can ask him to continue his investigation under the auspices of the legislative branch. And if Mr. Trump pardons himself, disregards court rulings or blatantly oversteps the boundaries of his legitimate authority in some other way, Congress should impeach him.

No flashing light will announce that the very survival of democracy is now at stake if Mr. Mueller is fired. And since nobody can say for sure that the Constitution will become toothless if congressional Republicans let yet another infraction pass, their instinct will be to defer their patriotic duty to some more opportune moment in the future. But that moment may never come. There may never be a time when we know for sure that this decision, today, will determine whether the American republic lives or dies.

In Hungary, democracy did not end when Mr. Orban staffed the electoral commission with his cronies, or when he put loyalists in charge of state television stations or even when he changed the Constitution to expand his powers. But now that he has taken all these steps, the opposition has little chance of ousting him at the next elections. Slowly but surely, Hungary has ceased to be a real democracy.

The temptation to delay opposing Mr. Trump until the right moment comes along is understandable. It’s also very dangerous. Even if congressional Republicans abdicate their duty, the Constitution may turn out to be unusually resilient. But the only sure way to save the Republic is for them to start standing up to the president’s authoritarian behavior — not next week, or next month, but today.