President Donald Trump's war on immigration is in full-blown mission creep. No longer does he only want to throw "bad hombres" out. He's even targeting immigrants who pose no security threat to the country. And recently, he has become preoccupied with so-called chain migration.

"CHAIN MIGRATION cannot be allowed to be part of any legislation on Immigration!" he bleated in one tweet. The practice is "horrible" and "bad for the country," he barked in others.

The president is using nativist language to trash a noble goal of America's immigration system: keeping families intact, which, as it happens, has also worked wonders for America's economy.

The term is meant to conjure images of a process in which one immigrant comes into the country and then pulls in hordes of relatives, who in turn pull in hordes more, until entire tribes and villages are emptied into the United States. The White House even released an infographic to that effect.

But that's not how things work. Beyond spouses and minor children, American law allows immigrants to sponsor only parents, adult children, or siblings—not aunts, uncles, and cousins. Moreover, they can do so only after they themselves receive green cards or become naturalized citizens.

According to a National Foundation for American Policy analysis of government data, it can take up to 45 years for an immigrant to gain entry and pull in the next link in the "chain." Using a typical case, the study pointed out that if a Mexican-American naturalized woman sponsored her married son from Mexico, it would take the son and his wife 20 years to get green cards. If the wife wished to bring her siblings over, the quickest route would be for her to become naturalized, too, which would take five years. Sponsoring them would take another 20, by which time they'd be middle aged!

As the Mercatus Center's Daniel Griswold wrote in The Hill, the U.S. system admits only 2–2.5 family members of immigrants per year for every 1,000 residents. That's the same rate as in Canada and Australia—countries whose skills-based systems are Trump's alleged models. And about 66 percent of the foreigners America admits under this category are spouses or minor children. Hence, America can't stop what nativists call "mass immigration" without breaking up nuclear families.

Sadly, even failed Republican presidential candidate Jeb Bush, an immigration dove and Mr. Family Values himself, recommended rebalancing America's immigration system by cutting the family-based immigration that accounts for two-thirds of all immigrants to the country, and ramping up the employment-based immigration that accounts for a third. To his credit, unlike Trump, Bush favored raising overall immigration levels. He knows that without more foreign workers, America's workforce will diminish by 8 million by 2035.

One reason Republicans such as Bush badmouth family-based immigration is that they believe allowing in relatives rather than skilled professionals is not good for the economy. That's understandable—and profoundly mistaken. Indeed, with the exception of aging parents, immigrants leave their home and hearth and come to America only if they believe they have a good shot at achieving something better here. Regardless of which bureaucratic category they're admitted under, they do what it takes to be productive and get ahead.

A study by Harriet Duleep of the College of William & Mary and Mark Regets, then of the National Science Foundation, examined three decades of census data and found no difference in the final earnings of foreigners sponsored by family members vs. those sponsored by employers. Even though the former make less money than the latter initially, they also make far bigger and more rapid income gains over time.

Why? Immigrants who come on family visas aren't tied to specific companies or occupations. They're freer to acquire new skills and go into fields where they expect good returns, achieving an even snugger fit with America's labor market than those who already had positions waiting for them. And since they don't have jobs in hand on arrival, they have a lower opportunity cost for starting a business, and are more likely to be able to count on family support. Finally, the "best and brightest" have many options, but they often pick America because they have family here.

"Chain migration" is clever vernacular that does double duty for nativists. It demonizes the family-based immigration process while masking the real agenda: breaking up foreign-born families so as to encourage immigrants here to "self deport" and discourage new ones from coming. It'll be a tremendous loss for America's economy if immigration prohibitionists succeed.