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For much of the past six months, I’ve tried to ignore the political ascent of Donald Trump. Too horrible to be true, I assured myself. But the unthinkable has happened, and Mr. Trump is now the likely nominee of the Republican Party. So what might a Trump presidency bring?

To be frank, the most horrifying aspects of Trumpism are not his economic policies. The worst he could do there would be to precipitate a global depression. It’s far scarier to contemplate an erratic, blustering demagogue taking command of the most powerful military force on earth. Or the foreign-policy calamities that might befall us.

But I’m an economist, so I’ll stick with economics.

Let’s start on familiar terrain: large tax cuts for the rich. Some variant of that theme has been a staple of Republican economic policy since Ronald Reagan. The evidence that “trickle down” doesn’t work began as a trickle but is now a flood. Never mind, the donor class wants it. So Mr. Trump has stood shoulder-to-shoulder with other Republican candidates in advocating tax cuts for the non-needy.

But Mr. Trump’s proposed tax cuts, naturally, have to be bigger. According to the Tax Policy Center, the 10-year net revenue loss from Mr. Trump’s proposals would be an astonishing $9.5 trillion—and that’s after netting out some $2 trillion in loophole-closers and other tax hikes that probably would never happen. The corresponding net revenue loss in Ted Cruz’s tax plan is $8.6 trillion. In case you can’t wrap your mind around numbers that large, $9.5 trillion is about 20% of all federal revenue projected for that decade.

Huge tax cuts would balloon the federal budget deficit, just as they did after the Reagan and Bush tax cuts. Yet Republicans decry even today’s comparatively small deficit. How does candidate Trump propose to square the circle? Well, we got an inkling in the March 3 debate, when he proposed to squeeze “hundreds of billions of dollars in waste” from a Medicare drug budget of $78 billion.

Next come Mr. Trump’s anti-immigration promises. First the candidate boasted that he could build a wall on the Mexican border for $8 billion; he has since upped it to $10 billion and cut the length of the wall in half. Did someone say “cost overruns?” Who cares? Mexico will pay, right? Wrong.

Second, has anyone told Mr. Trump that more Mexicans are going home than are coming here these days? So do we—does he—want to wall them in?

The wall is not the worst Trump immigration idea. Try to imagine the economic disruption and gigantic cost (not to mention the human tragedies) from rounding up 3% of the U.S. population (the undocumented) and deporting them. Who, by the way, will do the work when they’re gone?

But I have saved the worst for last: international trade. In TrumpWorld, America “loses” whenever we run a trade deficit. Really? Don’t Americans get goods and services at lower cost while our trading partners get pieces of paper like Treasury bills (our IOUs)? No one forces Americans to buy all those imports. Don’t voluntary exchanges benefit both parties to a transaction? Billionaires wouldn’t notice the higher prices at Wal-Mart, Costco and elsewhere, but middle-class Americans would.

If a trade deficit “costs American jobs,” how did the U.S. manage to have 4% unemployment in 2000 when the trade deficit was 3.7% of GDP—larger than today’s?

And remember, all the trade deficits and surpluses in the world must add up to zero. So if all trade deficits must disappear, so must all trade surpluses—which, in practice, would mean that international trade shrivels. We tried that approach in 1930 with the Smoot-Hawley tariffs. It didn’t work out too well.

Finally, let’s not forget that America is signatory to numerous international treaties. Mr. Trump says he’ll “break” the North American Free Trade Agreement if he can’t renegotiate it. Has he asked Canada or Mexico? Mr. Trump has also advocated 45% tariffs on Chinese imports. Has he heard of the World Trade Organization agreement? It goes on. And the costs of abrogating treaties transcend economics. What country wants to deal with a serial treaty breaker?

Let’s tote up the score. Mr. Trump’s tax plan is doable, if Congress is sufficiently pliant. But it would exacerbate income inequality and explode the federal deficit. Mr. Trump’s immigration plans are heartless, hugely expensive and incredibly disruptive to the U.S. economy. His positions on international trade display abysmal ignorance, are economically harmful and threaten America’s standing in the world.

And remember: These are the best parts of what Mr. Trump has to offer.