William Gale, one of the authors of the Tax Policy Center study that showed that Mitt Romney’s tax plan doesn’t add up, returns with a wonderful analogy to explain the numbers:

Suppose Governor Romney said that he wants to drive a car from Boston to Los Angeles in 15 hours. And suppose some analysts employed tools of arithmetic to conclude that "If Governor Romney wants to drive from Boston to LA in 15 hours, it is mathematically impossible to avoid speeding." After all, the drive from LA to Boston is about 3,000 miles, so to take only 15 hours would require an average of 200 miles per hour. Certainly other road trips are possible — but the particular one proposed here is not.

The parameters Romney has supplied were originally a 20 percent rate cut; certain other specific tax treatments; and revenue neutrality. What Gale and his colleagues showed was that the only way to make Romney’s plan work under those constraints was to raise taxes on middle-class taxpayers.

What we’ve seen since is a blizzard of misdirection, but nothing, as Gale says, has changed to make the math work. The only thing that has changed is that Romney now adds that taxes won’t go up on the middle class. But that only makes it worse; given that additional constraint, it’s even more clear than ever that something has to give. Either the rate cuts have to be lower, the other specific tax breaks have to be reduced or ignored, middle-class taxes have to go up – or, what I suspect is by far the most likely if Romney actually wins and has a Congress willing to do what he wants, revenue will just plummet and the deficit explode. But that’s just a guess based on past performance; all we do know is that, as a matter of addition and subtraction, Romney is not leveling with us about something.

The rest of it, again, is misdirection. Last week’s Romney float of a plan to cap deductions for the wealthy? Interesting as policy – but totally irrelevant to the question at hand, because the Tax Policy Center said there weren’t enough deductions to make back the revenue even if the deductions were all eliminated. The studies that Romney trots out to support him? They didn’t get to the core issue that the center raised.

The math is the math. There just aren’t enough tax expenditures that the wealthy use to allow Romney to cut tax rates by 20 percent without either reducing revenue or by making it up with tax increases on everyone else, at least not if he also retains the other specific provisions he supports. How to characterize that situation – Romney and Barack Obama clashed on whether it’s a “$5 trillion tax cut” – is a matter of semantics. But the basic situation is neither semantics nor opinion; it’s just arithmetic.