By Froma Harrop

Let us avert our eyes from the president’s display of instability in Phoenix. We knew what to expect. When Donald Trump occasionally goes “presidential,” as he did the day before on Afghanistan, a tantrum is sure to follow. You could set your watch by it.

Many Republicans are quietly planning for a post-Trump era, when public debate may again center on policies and not on wacky politics. Democrats must do so, as well, and a good start would be an issue on which Trump has great traction with the public. That would be immigration.

Republicans oppose illegal immigration. Polls show that Democrats and independents, though not so obsessed over unlawful immigration, are plenty concerned about it, too. The operative word here is “illegal,” but the entire program needs a going-over.

The labor market is a market, and the entry of large numbers of low-skilled workers can’t help but reduce the prospects for low-skilled natives and immigrants here legally. The cheap-labor right and diversity left often go into contortions dismissing the laws of supply and demand — and fail.

George Borjas of Harvard offers the most clear-eyed view of mass immigration’s impact. “The influx of immigrants can potentially be a net good for the nation, increasing the total wealth of the population,” he writes. But that wealth is not evenly divided. “Somebody’s lower wage is always somebody else’s higher profit.”

Immigrants admitted in the past two decades have increased the size of the low-skilled workforce by 25 percent, according to Borjas’ research. As a result, annual earnings by high-school dropouts have fallen by between $800 and $1,500 a year.

Bernie Sanders gets this. And so did Barack Obama. In fact, federal immigration officials under Trump are removing fewer of the undocumented than they did during the slowest years of Obama’s presidency, Politico reports. Arrests have soared under Trump, but arrests don’t equal deportations.

One reason for this may be that fewer people are trying to sneak across the border in the wake of Trump’s harsh rhetoric. But suffice it to say, Obama was on the case. In addition, Obama faced opposition from diversity advocates in his own party, who attacked him as “deporter in chief.”

Americans across the political spectrum generally support a generous immigration program, as long as it’s orderly. Trump’s disordered responses— from his ethnic smears to plans for wasting colossal sums of taxpayer money on a wall with Mexico — give Democrats a great opportunity to support humane and more intelligent alternatives.

They may start by giving a respectful hearing to a new Senate Republican proposal for immigration reform. A points-based system favoring immigrants with needed skills makes great economic sense. Australia and Canada do it.

That does not preclude admitting a good number of low-skilled people rich in ambition and work ethic. Just adjust the numbers for changing economic conditions.

“Chain migration” now accounts for two-thirds of legal immigration and needs curbing. Of course, immigrants’ spouses and young children should be admitted. But rules prioritizing extended family members reduce opportunities for those from other places and possessing higher skills.

Canadians also have mixed feelings about their large immigration program, but enforcement of the law helps maintain support for it. When more than 3,000 people, mostly Haitian asylum-seekers, crossed into Canada from the United States in July, Prime Minister Justin Trudeau warned against entering the country “irregularly.”

“There are rigorous immigration and customs rules that will be followed,” he said. “Make no mistake.”

Democrats must also make no mistake that immigration rules will be followed. And their politicians need not fear accusations of racism from some on the left (and right). They can thank Trump for giving them space.

Follow Froma Harrop on Twitter @FromaHarrop. Email her at fharrop@gmail.com.