He wants to balance the budget—in fact, he wants to amend the Constitution to make that mandatory—but at the same time he wants to cut taxes by $4 trillion or so, increase defense spending, and keep antipoverty spending where it is. That doesn’t leave a lot of places to find savings. There probably aren’t any in non-defense discretionary spending—things like roads and research—when it’s already at a 40-year low. So you’d have to get them all by cutting Social Security and Medicare, and cutting them now. Rubio, though, only wants to “reform” entitlements for future seniors, not current ones. And that leaves you with big, fat deficits for a good, long while.

This is the same problem Republicans have had for 35 years now. That’s how long they’ve been running on deficit-financed tax cuts and fiscal responsibility. So Rubio, who’s trying to portray himself as a new kind of Republican, is seizing on the only thing that makes that combination work: saying tax cuts will pay for themselves. Specifically, he’s said that the way to balance the budget is with “dynamic economic growth,” which, of course, his tax cuts are supposed to provide. That’s saying you can eat your cake and have it too, because eating it will make more of it appear. But anyone who remembers the big deficits that came after Ronald Reagan and George W. Bush’s big tax cuts knows that’s not the case. Indeed, former Bush adviser Greg Mankiw knew this himself, calling people who said otherwise little more than “charlatans and cranks.”