I start with James Madison, who was as committed as either Locke or Smith to the supremacy of individual liberty but was acutely aware of the inherent tension between liberty and democracy. Madison recognized that “the people” can be as tyrannical as a monarch. It is not enough, he wrote in Federalist 51, “to guard the society against the oppression of the rulers”; it is equally necessary “to guard one part of the society against the oppression of the other part.” What we now call a “liberal democracy” is a state where majoritarianism is tempered by a respect for individual rights as well as the rights of political or ethnic minorities.

The opposite of liberalism is not conservatism, as we understand the term in modern America, but populism. The illiberal populism of Donald Trump, Hungary’s Viktor Orban or Brazil’s Jair Bolsonaro, each of whom was more or less fairly elected, entails the cynical and reckless use of the mechanisms of democracy to disenfranchise political minorities, politicize the state, stigmatize immigrants and other “outsiders,” and diminish civil liberties. Populists address an exclusionary “us.” As Mr. Trump himself once put it with rare clarity, “The only important thing is the unification of the people — because the other people don’t mean anything.” Populism is democracy without liberalism.

We suffer from a confusion of terms, and not only in regard to liberalism. Elizabeth Warren and Bernie Sanders are routinely referred to as “populists,” including in this newspaper, presumably because they assail the rich. But so did Theodore Roosevelt. In his day, as in ours, the rich vacuumed up almost all the value produced by the economy. Does pointing that out make you a populist? Does denying that claim, or endorsing the virtues of gross inequality, mean that you are not a populist? The thought is absurd. None of the Democratic candidates would talk about, or treat, “the people” as Mr. Trump has done. They are not populists. Are they liberals? Bernie Sanders — who calls himself a socialist, opposes free trade and shows very little regard for the working of the free market — is not. I have my doubts about Elizabeth Warren. But they’re not populists either.

Today, paradoxically, it is not liberals but conservatives who endanger liberalism. The secular, free-market conservatives who voted for Ronald Reagan accepted the basic apparatus of a liberal society. But Mr. Reagan also welcomed into the party extremists and conservative evangelicals who thought about politics in far more absolute terms. Over time, those groups took over the host body of the Republicans, leaving us with an illiberal party that represents half the country. Today’s Republican leaders, with the vehement support of their base, have threatened the autonomy of the federal and state judiciary, treated intelligence and law-enforcement agencies as instruments of the “deep state,” attacked the press and pulled down the empire of science, reason and fact into the mire of “fake news.”

It may be that a combination of the impending impeachment proceedings against Mr. Trump and a Democratic victory in 2020 will break the illiberal fever. Certainly any imaginable Democratic victor would put an end to attacks on the government and the press. But it is not, in the end, the formal rules and institutions that sustain liberal democracy. It is the widespread faith in them, what we call “norms.” Forty years of swelling illiberalism on the right — and some reciprocal illiberalism on the left — have deeply corroded that faith.