President Trump began his campaign assailing immigrants as ruthless lawbreakers who steal American jobs with impunity. To halt them, he has vowed to build a wall along the border with Mexico, hire thousands of new immigration agents, ramp up immigrant detention and subject visa applicants to even more rigorous vetting. His administration has been largely silent, however, about the strongest magnet that has drawn millions of immigrants, legal and not, to the United States for generations: jobs.

American employers continue to assume relatively little risk by hiring undocumented immigrants to perform menial, backbreaking work, often for little pay. Meanwhile, as Mr. Trump’s deportation crackdown accelerates, families are being ripped apart, and communities of hard-working immigrants with deep roots in this country are gripped by fear and uncertainty. As long as employers remain off the hook, a border wall and an expanded dragnet can only make temporary dents in the flows of undocumented immigrants.

There was widespread, bipartisan acknowledgment of this reality the last time lawmakers managed to pass a comprehensive immigration bill. In 1986, President Ronald Reagan signed into law the Immigration Reform and Control Act, which outlined a carrots-and-sticks approach to reducing the number of undocumented workers in the American labor force. The bill offered a pathway to citizenship to roughly 2.7 million people who had been living in the United States without permission, set in motion an era of tighter controls at the border and made knowingly hiring undocumented people a crime.