Marco Rubio's toughest political opponent may be math.
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."
But that hasn't stopped the usual supply-side suspects, like the right-leaning Tax Foundation, from claiming that Rubio's trillions of dollars of tax cuts would, after nine years of bigger deficits, actually make tax revenues go up after that by supercharging growth so much. If this sounds like wishful thinking masquerading as actual analysis, well, you're in touch with empirical reality. "This would not pass muster," economist Laurence Kotlikoff told the New York Times's Josh Barro, "as an undergraduate model at a top university." Though those last four words might not be necessary.
It's a case of fiscal dissonance. Republicans think tax cuts are the solution to everything, but they also think the debt is one of our biggest problems. The only way to make this compute is to cut spending more than you cut taxes. That's what Paul Ryan, for example, has proposed in his budgets that would cut safety net and non-defense discretionary spending down to almost nothing. The problem here, though, is that specific government programs are a lot more popular than government in the abstract. Voters, in other words, might say that they want smaller government, but not if that means, well, actually making the government smaller by slashing Medicaid and R&D spending. Besides, it's bad politics to make the trade-off between more tax cuts for the rich and less spending for the poor more explicit. That would make a lot of voters think the Republicans don't care about people like them, which Rubio doesn't want to doom the party in 2016 like it did in 2012.
So Rubio's taken what Jim Pethokoukis of AEI calls "the Oprah theory of tax cuts"—you get a tax cut, and you get a tax cut, and we all get a tax cut—and applied it to spending too. Everybody gets what they want today, and all the sacrifices come, say, eight years down the road when Rubio would be out of office. It's what George W. Bush did.
You can't repeal and replace math.