Mitt Romney added an important detail to his tax plan yesterday, and if Obama doesn't jump all over this tonight, well, there'd be no excuse for it.

In an interview yesterday, Romney envisioned one possible way of paying for his tax cuts, a limit on deductions of $17,000. That is to say, all of your deductions--state and local taxes, mortgage interest, home repair, charitable contributions--would be capped at $17,000.

There is some merit to this general category of idea. Lots of experts despise the home mortgage interest deduction because it helps the better off, discriminates against those who don't own homes, and so forth. In a perfect world, designing a tax system from scratch, one would probably not want to include it. However, it exists, and millions of people rely on it.

I have to speak to more people and study up on this more, but my preliminary instincts and conversations with wonks suggest to me that the Romney cap, or "bucket" as he calls it, would just whack the upper middle class. It probably wouldn't impact people who make the median salary of $50,000, because most people at that level probably don't itemize to begin with. So in that sense there's a progressive element to it.

But this change would crush households making $150,000 or $200,000. For example, a couple with a $400,000 mortgage, say, pays upwards of $10,000 a year in mortgage interest alone. Throw in state and local taxes and you hit $17,000 pretty quick. So it creates a situation where one taxpayer gets to deduct her contributions but another one doesn't get to deduct his because he has a bigger mortgage. And lots of things like that.

And while it has a progressive element, it also would be really regressive and would benefit the rich enormously, especially those whose greater share of income comes from capital gains. So this is kind of shocking, politically, because these taxpayers, the upper middle class, are the one group no politician ever wants to touch or inconvenience in any way.

There's still no way to cut rates by 20 percent, cut or maintain current capital gains rates, and now impose this cap without it hitting upper-middle-income people, which I'd define as roughly $120,000 to $250,000. Their taxes will go up in this scenario almost certainly.

Maybe Mitt floated this the day before the debate so the Obama people wouldn't really have time to run numbers. They'd better be.