Every political campaign involves a choice between elevating political discourse and degrading it. The 2016 election brought a pornographic film star into prime time and made “pussy” front-page news. How it could get any worse in 2020 is difficult to imagine. But the problem isn’t the word “pussy” and the pornification of politics, however demeaning; the problem is the word “nationalism” and the abandonment of liberalism.

“I’m a nationalist, O.K.?” President Trump said at a rally in Houston last year. “Use that word.”

Please do not use that word. But please do use the word “nation” — the nation of the Gettysburg Address, “a new nation, conceived in liberty, and dedicated to the proposition that all men are created equal” — and please do use the word “liberalism,” which is what Lincoln meant by that proposition.

Candidates who swat at Mr. Trump’s tweets like so many black flies will only find themselves eaten alive. But anyone running in 2020 who is willing to ignore the flies has an opportunity to speak with clarity and purpose about what’s at stake: the liberal nation-state itself.

The United States is a nation founded on a deeply moral commitment to human dignity. All of us are equal: We are equal as citizens and we are equal under the law. Notwithstanding the agony and hypocrisy of the nation’s past and the cruelty and pettiness of its present, these truths endure, in the form of liberalism. Liberalism is not a species of partisanship. Liberalism is the belief that people are good and should be free and that people organize governments in order to guarantee that freedom. That guarantee includes protecting a habitable planet.