Instead of mobilizing international coalitions to take on world problems, he touts the doctrine of “every nation for itself” and has led America into isolated positions on trade, climate change and Middle East peace. Instead of engaging in creative diplomacy, he has insulted United States neighbors and allies, walked away from key international agreements, mocked multilateral organizations and stripped the State Department of its resources and role. Instead of standing up for the values of a free society, Mr. Trump, with his oft-vented scorn for democracy’s building blocks, has strengthened the hands of dictators. No longer need they fear United States criticism regarding human rights or civil liberties. On the contrary, they can and do point to Mr. Trump’s own words to justify their repressive actions.

At one time or another, Mr. Trump has attacked the judiciary, ridiculed the media, defended torture, condoned police brutality, urged supporters to rough up hecklers and — jokingly or not — equated mere policy disagreements with treason. He tried to undermine faith in America’s electoral process through a bogus advisory commission on voter integrity. He routinely vilifies federal law enforcement institutions. He libels immigrants and the countries from which they come. His words are so often at odds with the truth that they can appear ignorant, yet are in fact calculated to exacerbate religious, social and racial divisions. Overseas, rather than stand up to bullies, Mr. Trump appears to like bullies, and they are delighted to have him represent the American brand. If one were to draft a script chronicling fascism’s resurrection, the abdication of America’s moral leadership would make a credible first scene.

Equally alarming is the chance that Mr. Trump will set in motion events that neither he nor anyone else can control. His policy toward North Korea changes by the day and might quickly return to saber-rattling should Pyongyang prove stubborn before or during talks. His threat to withdraw from the 2015 Iran nuclear agreement could unravel a pact that has made the world safer and could undermine America’s reputation for trustworthiness at a critical moment. His support of protectionist tariffs invites retaliation from major trading partners — creating unnecessary conflicts and putting at risk millions of export-dependent jobs. The recent purge of his national security team raises new questions about the quality of advice he will receive. John Bolton starts work in the White House on Monday.

What is to be done? First, defend the truth. A free press, for example, is not the enemy of the American people; it is the protector of the American people. Second, we must reinforce the principle that no one, not even the president, is above the law. Third, we should each do our part to energize the democratic process by registering new voters, listening respectfully to those with whom we disagree, knocking on doors for favored candidates, and ignoring the cynical counsel: “There’s nothing to be done.”

I’m 80 years old, but I can still be inspired when I see young people coming together to demand the right to study without having to wear a flak jacket.

We should also reflect on the definition of greatness. Can a nation merit that label by aligning itself with dictators and autocrats, ignoring human rights, declaring open season on the environment, and disdaining the use of diplomacy at a time when virtually every serious problem requires international cooperation?

To me, greatness goes a little deeper than how much marble we put in our hotel lobbies and whether we have a Soviet-style military parade. America at its best is a place where people from a multitude of backgrounds work together to safeguard the rights and enrich the lives of all. That’s the example we have always aspired to set and the model people around the world hunger to see. And no politician, not even one in the Oval Office, should be allowed to tarnish that dream.