A MAJOR theme of the Republican convention so far has been that a vote for Donald Trump is a vote to restore America’s position in world affairs, which President Obama has, in the GOP view, deliberately undermined. Mr. Trump “will rebuild our military and stand with our allies,” vice presidential nominee Mike Pence proclaimed. He also said: “We cannot have four more years of apologizing to our enemies and abandoning our friends.”

In reality, Mr. Trump’s political rise has already disturbed traditional American allies and encouraged adversaries such as Russia, because of the candidate’s own disparagement of U.S. security alliances with Europe, Japan and South Korea. And in the midst of the Cleveland get-together — nearly overlapping with his running mate’s remarks, in fact — Mr. Trump was giving an interview to the New York Times that was giving the lie to his supporters and reinforcing doubts about his potential policies abroad.

Stand with our allies? No, Mr. Trump told the Times, we “always have to be prepared to walk,” lest they rip us off for the costs of defending them. In his transactional world, there is no such thing as a long-term U.S. investment that pays for itself many times over in global stability. Chillingly, Mr. Trump even seemed to place conditions on NATO’s ironclad mutual security guarantee, saying he would honor it in the event of a Russian attack on the Baltic states if the victim of that attack had, in his opinion, “fulfilled their obligations to us.”

This is an extraordinary willingness to question 70 years of bedrock political consensus on U.S. foreign policy — one that includes Mr. Obama, who has dispatched U.S. troops to defend the Baltics. Of course, there is no such thing as an unquestionable policy. What’s astonishing about Mr. Trump, though, is the obvious casualness with which he muses about such matters — as if the words of even a potential commander in chief do not influence world affairs the moment they are uttered.

But they do. Equally remarkable was Mr. Trump’s unilateral surrender of the moral high ground when it comes to the global cause of democracy and human rights. Republicans love to denounce Mr. Obama for allegedly going on “apology tours” around the world. Never in his most self-critical moments, however, has Mr. Obama failed to assert the United States’ right to defend democracy abroad based not on our perfection but on our willingness as a people to pursue betterment.

Mr. Trump, by contrast, said this, apropos the undeniably ugly events in cities such as Baltimore and Ferguson, Mo., of late: “When the world looks at how bad the United States is, and then we go and talk about civil liberties, I don’t think we’re a very good messenger. . . . I don’t know that we have a right to lecture.” This is music to the ears of dictators everywhere, from Xi Jinping of China to Recep Tayyip Erdogan of Turkey, whose sweeping crackdown in the wake of a failed coup was the specific context Mr. Trump addressed.

Where Ronald Reagan saw his country as a shining city on a hill, Mr. Trump, apparently, perceives the moral equivalent of a low-rent district.

Contrary to his apologists in Cleveland, Mr. Trump does not understand the value of American global leadership or the moral ground upon which it ultimately rests. If he becomes president, the retreat of U.S. influence for which Republicans now hold Mr. Obama culpable will likely turn into a rout.