So how would TrumpCare operate? How many Americans would be eligible? What would it cost? It isn’t enough for Mr. Trump to go on “60 Minutes” and claim, “They can have their doctors, they can have plans, they can have everything.” Americans have heard empty promises before. They want an actual proposal that works.

There are already huge gaps between what Mr. Trump says and reality. For instance, he has complained about “hedge fund guys that are making a lot of money that aren’t paying anything” in taxes. Yet when he released his tax plan, he proposed dropping the top rate on hedge fund fees to 25% from 39.6%. True, the plan would raise taxes on their performance bonuses to 25% from 23.8%. But the result would be a generous tax cut provided to people Mr. Trump claims pay no taxes despite “making a hell of a lot of money.”

What about entitlements? “I’m not going to cut Social Security like every other Republican,” Mr. Trump says. “And I’m not going to cut Medicare.” But the Social Security Trust Fund will go bankrupt by 2034, and the Medicare Hospital Insurance Trust Fund will be exhausted in 2030. The only way the next president can save these entitlements is to reform them and help America avoid a debt crisis.

Mr. Trump has proven he can do outrageous. But soon Mr. Trump must demonstrate that which he has so far avoided: substance.