Indeed, the cost of capping the rate on pass-through income is breathtaking. The Tax Policy Center and Tax Foundation both estimate that Mr. Trump’s proposed cap would cost about $1.5 trillion over 10 years, about a quarter of the net cost of his campaign tax plan. After accounting for ensuing efforts to game the loophole, the Tax Policy Center and Goldman Sachs estimate it would cost $2 trillion or more.

To make matters worse, the proposal could destabilize funding for Medicare and Social Security, effects that aren’t included in these estimates. As more people claim their labor income as business income, the Medicare and Social Security Trust Funds could collect less payroll tax revenue. Together with the pressure that the proposal’s cost would place on our long-term fiscal outlook, this could jeopardize benefits for future retirees.

Treasury Secretary Steven Mnuchin has said he can prevent gaming of the special rate cap, but there’s no practical way to do so for the wealthy. The most prominent anti-avoidance proposal, by former Representative Dave Camp, a Michigan Republican, is simply a backdoor formula that replaces the headline preferential rate for pass-through income with a somewhat less preferential effective rate — essentially conceding that the only way to limit gaming is not to enact a preferential rate in the first place.

Rather than falsely claiming that a cap on the pass-through rate would benefit small businesses, some advocates argue that it’s only fair to treat all businesses the same. If Congress succeeds in cutting the corporate tax rate to 15 percent as the president has proposed, shouldn’t the same rate apply to pass-throughs?

But this argument also falls apart under the facts. Pass-throughs already enjoy a large tax bias, after accounting for investor-level taxes. That’s why we’ve seen dramatic growth in their numbers, to 60 percent of net business income today from less than one-fourth in 1980. What’s more, pass-throughs that prefer to pay tax as C corporations can change their tax treatment relatively easily at any time, whereas public C corporations generally have to go private to become pass-throughs.

Achieving tax fairness is an admirable goal, and Congress should pursue it. But to get there, it should try to close loopholes, not open giant new ones.