This month, Hillary Clinton announced a plan to make public colleges free for the children of any family earning less than $125,000 a year. The move was widely seen as an appeal to supporters of her primary opponent Bernie Sanders, who made free college a pillar of his insurgent campaign.

Binyamin Appelbaum of The New York Times wrote Saturday that the plan could have the perverse effect of driving tuition higher. As is often the case with campaign promises, the details are fuzzy. And probably for good reason. A look at how states finance higher education shows that the more Mrs. Clinton tries to make good on her free college promises, the more problematic the policies needed to get there become.

It’s easy to devise a straightforward way to meet Mrs. Clinton’s goals: Students at public colleges and universities simply forward their tuition bills to the federal Department of Education, which pays them. Congress could write such a law on a single piece of paper and still have time for golf in the afternoon.

That policy would also be a financial disaster. Any public university president with an ounce of sense would simply raise annual tuition by $5,000 or $10,000 or more, secure in the knowledge that Uncle Sam would foot the bill. It would be an unlimited bonanza for colleges and a huge drain on the Treasury, for a program already estimated to cost tens of billions of dollars per year.