So Vladimir Putin, the Russian president, has told The Financial Times, “The liberal idea has become obsolete.” It has, he says, “outlived its purpose.” President Trump finds him amusing. In Japan, at the G-20 summit meeting, he wags his finger at Putin and says, with a grin: “Don’t meddle in the election, President.”

It’s a measure of American decline over the past three decades that Putin’s claim, however objectionable, is no longer preposterous and Trump’s frivolity no longer surprising. Thirty years ago, the Berlin Wall fell. Two years later, the Soviet Union was gone. The liberal idea was triumphant, and American power virtually uncontested in “the unipolar moment.”

That moment was heady. Liberalism posited the indivisibility of freedom and human dignity, as well as the idea that the rule of law and democracy offered the best chance for human advancement, peace and prosperity. Its spread appeared inevitable and irreversible. Its guiding spirit was the United States.

The road from that high-water mark of the American idea to Trump’s autocrat-coddling indecency offers a story of squandered American opportunity and eroded American self-belief that Edward Gibbon would have qualified as “decline and fall.” All Democratic candidates should be asked what they intend to do about it.