Fox News' Chris Wallace pressed GOP vice presidential candidate Paul Ryan on Sunday morning to try to explain how Mitt Romney's tax plan would add up. The conundrum is that Romney is proposing to lower all individual rates by 20 percent by targeting deductions for high-income earners, without putting the burden on the middle class or adding to the deficit—a plan the Tax Policy Center has shown to be "mathematically impossible."

Wallace tried to get Ryan to explain how the tax plan achieved all of these goals and pointed out that the plan was "not revenue neutral unless you take away the deductions," but the Wisconsin Republican deflected the question:

RYAN: We’re saying, limited deductions so you can lower tax rates for everybody. Start with people at the higher end...lowering tax rates by broadening the tax base works. 

WALLACE: You haven’t given me the math.

RYAN: (laughs) Well, I don’t have the time. It would take me too long to go through all the math. But let me say it this way, you can lower tax rates by 20 percent across the board by closing loopholes and still have preferences for the middle class for things like charitable deductions, for home purchases, for health care...

WALLACE: If — just suppose — that the doubters are right, President Romney takes office and the math doesn’t add up… 

RYAN: First of all, we've run the numbers. I've run them in Congress, they do*. We've got five other studies that show you can do this.

So the basic message from Ryan here is: Trust us—the math adds up.

The problem is, even one of the studies that Romney has cited to justify his plan—which Ryan refers to in passing above—shows that his tax plan won't work unless taxes go up for those making over $100,000, which Romney still counts as middle-class. Another study that the Romney camp cites, by Princeton's Harvey Rosen, uses implausibly optimistic assumptions about economic growth, as my colleague Dylan Matthews explains here.

*Update: This quote originally ran as "First of all, run the numbers. They've run them in Congress." It's been corrected, per the official Fox transcript.