Just in time for the midterms, a caravan of migrants, thousands strong, is wending its way north from Central America toward the U.S. border. President Donald Trump is on the attack. And, lo, it’s time for another national conversation about immigration.


As usual, that conversation has started unproductively, with Trump’s declaration - on no apparent evidence - that there are criminals and “unknown Middle Easterners” mixed in with the migrants. The conversation might easily end unproductively, too, with Democrats condemning his unsubstantiated claims, then hastily changing the subject to something else.


But if that’s all, just the usual hackneyed, base-pleasing rhetorical points lobbed across the ideological divide, that would be a shame.


Trump is of course wrong to cast fact-free aspersions on desperate people seeking a better life. But the left is wrong if it thinks that making that observation ends the argument, because even a caravan of nothing but decent, hard-working people would raise big questions. There are billions of decent, hard-working people living in the world. Do all of them have a right to migrate to the United States merely because doing so would make them better off?


Immigration restrictionists and advocates both struggle with that question to some degree, depending on the scale they use when considering it. On the macro scale, restrictionists find it easy to say, no, of course we can’t let in every single person who wants to come, because doing so would transform the country into one that few Americans of any political persuasion would want to live in.


The United States would be vastly more unequal, in income and opportunity. Migrants would languish in the kind of squalid poverty that the United States hasn’t seen for a century, in numbers - a doubling of the U.S. population is probably a conservative estimate - that would defeat any conceivable tax-and-transfer program. When you’re working with long strings of zeroes, the restrictionist position seems overwhelmingly compelling.


But on the micro scale, where you can see the human faces, “reasonable” immigration restriction starts looking more like “ruthless.” Those Honduran migrants lying under a tarp in Huixtla, Mexico, aren’t any less deserving than you are of clean drinking water, safe streets and a warm, dry abode. They just failed to hit the Pick-6 in the birthplace lottery. By what moral right do we tell them to go back to Tegucigalpa and hope that their local MS-13 chapter decides to prey on somebody else? Looking at actual faces, rather than zeroes, many people would answer: “None.”


A truly open-borders policy has almost no constituency in the American electorate, but it’s the implicit philosophical framing of most media stories on immigration, which tend to focus on the micro problems of individuals, not the macro problem of just how many of those individuals the country can admit. Trump’s anti-immigration platform may have been so successful because he has fought back on the micro scale, conjuring up menacing criminals and terrorists, instead of retreating to statistics and white papers.


Politically, his tactic pleases his base; morally and intellectually, it is dubious, to say the least. If we’re going to find any sort of sane, workable immigration compromise, we don’t need more anecdotes; we need to synthesize the micro and macro problems.


The best such synthesis I’ve seen in recent years comes in Reihan Salam’s new book, “Melting Pot or Civil War?” Himself the son of Bangladeshi immigrants, Salam is obviously sympathetic to their plight. But he’s also clear-eyed about the difficulties of absorbing all those who would like to enter an economy with a steadily declining demand for unskilled labor.


As the title suggests, Salam wants an America that welcomes immigrants - but he also wants an America that integrates them and doesn’t just import a new underclass. That means limiting the flow of low-skill immigrants and shifting to higher-skill migrants who arrive ready to thrive.


You don’t have to like his proposals, and many won’t. Salam readily acknowledges that a shift toward higher-skill migration means that many of the world’s neediest people will be left in poverty outside the United States. That’s a hard bullet to bite, and many won’t want to do it.


But even if you disagree with Salam, he shows an admirable and all-too-rare willingness to lay out the problem in clear terms, and to acknowledge the real human costs that any immigration policy inevitably involves. Given the caravan heading for the U.S. border and the much larger number of people who will follow if it succeeds, that’s the immigration conversation we actually need to have, and soon.


Megan McArdle is a columnist with The Washington Post.