Whatever his gyrations, Mr. Trump always does make clear where his heart lies — with the anti-immigrant, nativist and racist signals that he scurrilously employed to build his base.

He used the shameful “birther” campaign against President Obama’s legitimacy as a wedge for his candidacy. But then he opportunistically denied his own record, trolling for undecided voters by conceding that Mr. Obama was a born American. In the process he tried to smear Mrs. Clinton as the instigator of the birther canard and then fled reporters’ questions.

Since his campaign began, NBC News has tabulated that Mr. Trump has made 117 distinct policy shifts on 20 major issues, including three contradictory views on abortion in one eight-hour stretch. As reporters try to pin down his contradictions, Mr. Trump has mocked them at his rallies. He said he would “loosen” libel laws to make it easier to sue news organizations that displease him.

An expert negotiator who can fix government and overpower other world leaders?

His plan for cutting the national debt was far from a confidence builder: He said he might try to persuade creditors to accept less than the government owed. This fanciful notion, imported from Mr. Trump’s debt-steeped real estate world, would undermine faith in the government and the stability of global financial markets. His tax-cut plan has been no less alarming. It was initially estimated to cost $10 trillion in tax revenue, then, after revisions, maybe $3 trillion, by one adviser’s estimate. There is no credible indication of how this would be paid for — only assurances that those in the upper brackets will be favored.

If Mr. Trump were to become president, his open doubts about the value of NATO would present a major diplomatic and security challenge, as would his repeated denunciations of trade deals and relations with China. Mr. Trump promises to renegotiate the Iran nuclear control agreement, as if it were an air-rights deal on Broadway. Numerous experts on national defense and international affairs have recoiled at the thought of his commanding the nuclear arsenal. Former Secretary of State Colin Powell privately called Mr. Trump “an international pariah.” Mr. Trump has repeatedly denounced global warming as a “hoax,” although a golf course he owns in Ireland is citing global warming in seeking to build a protective wall against a rising sea.

In expressing admiration for the Russian president, Vladimir Putin, Mr. Trump implies acceptance of Mr. Putin’s dictatorial abuse of critics and dissenters, some of whom have turned up murdered, and Mr. Putin’s vicious crackdown on the press. Even worse was Mr. Trump’s urging Russia to meddle in the presidential campaign by hacking the email of former Secretary of State Clinton. Voters should consider what sort of deals Mr. Putin might obtain if Mr. Trump, his admirer, wins the White House.

A change agent for the nation and the world?

There can be little doubt of that. But voters should be asking themselves if Mr. Trump will deliver the kind of change they want. Starting a series of trade wars is a recipe for recession, not for new American jobs. Blowing a hole in the deficit by cutting taxes for the wealthy will not secure Americans’ financial future, and alienating our allies won’t protect our security. Mr. Trump has also said he will get rid of the new national health insurance system that millions now depend on, without saying how he would replace it.