Let’s assume, for fun, that Donald Trump’s supporters are thinking with their brains, not their viscera. If so, they will want to know that the issue he has lassoed for self-aggrandizement has been utterly demagogued. Trump is playing them for chumps.

A young woman was murdered by an undocumented immigrant in a so-called “sanctuary city.” Awful, of course. San Francisco’s officials bear some of the blame for Kate Steinle’s death by declining to enforce the law.

But Trump has abused the trust of his audiences by suggesting that an illegal-immigrant crime spree is the great threat to our nation. Trump admirers yearn to “take our country back.” From whom? From Barack Obama, Valerie Jarrett, Al Sharpton, The New York Times, Jon Stewart and Hillary Clinton? Or from illegal immigrants from Mexico?

The United States is very much in decline, but the role of illegal immigration in that slide is negligible. Our lack of economic growth, our withdrawal from world leadership, the decline of work and the rise of dependency on government, law-flouting by those in power, the degradation of our entertainment culture, rent-seeking by entrenched interests, the stultifying politicization of education (especially higher education) — to say nothing of the nuclearization and enrichment of the world’s worst terror state — those are the great challenges we face.

Obama’s flagrantly illegal waiver for illegal immigrants living here made everyone who values the rule of law see red. And yet, a little perspective is in order.

Illegal immigration is declining. Between 1990 and 2007, the number of illegal immigrants tripled. In 2000, an estimated 1.6 million illegals entered the United States. Since 2012, that number has dropped to about 400,000 — even accounting for the flood of underage migrants last year.

We’ve built fences along all but the most inaccessible areas along the border. Over the past decade, we’ve spent $10.7 billion on fences, cameras and other measures, including doubling the number of border patrol agents to 18,000. We’ve also spent billions on biometric identity management and other things, bringing the total expenditure for border control to $16.2 billion last year. Those truly serious about ending illegal immigration altogether must grapple with national identity cards. Another fence isn’t going to do it.

The population of illegal immigrants here is aging, which suggests that fewer young people are making the increasingly treacherous journey across the desert.

As for the epidemic of crime for which illegal immigrants are said to be responsible — it’s a myth. Crime rates have declined as immigration has increased.

Having stoked rage about illegal immigrants, Trump now urges that after deporting 11 or 12 million people, he will “let the good ones come back,” force Mexico to pay for a “wall” and “impound remittances” from “illegal wages.” A candidate for student council president could devise more serious solutions than those. But then, student council types tend to be in earnest. Trump is simply on the ultimate ego trip.