America's days as the exceptional nation seem to be over. At home, President Donald Trump is weakening the foundations of representative government. Abroad, he has set aside democracy promotion and seems more inclined to work with autocracies like China and Russia than with America's traditional democratic allies.

While Mr. Trump's assault on American democracy is both without precedent and dangerous, he is right to hold off on the promotion of democracy abroad and to reach across ideological dividing lines. Improving relations with China, as Mr. Trump will aim to do at his summit with Xi Jinping on Thursday, is a must in an interdependent world.

Yet Mr. Trump's dismissal of democracy promotion is flawed in one critical respect: By compromising America's own democracy and backing illiberal populists abroad, he is extinguishing the nation's founding mission to change the world by example. The Puritan leader John Winthrop envisioned a "city upon a hill" in 1630, changing the world by serving as a democratic model. As Thomas Paine wrote in 1776, "We have it in our power to begin the world over again."

The United States is squandering its ability to bend the arc of history toward more freedom. This patient approach to spreading democracy urgently needs resuscitation.