Mitt: I'm not releasing tax returns because John Kerry's wife, that's why

Fox & Friends threw Mitt Romney a retroactive retirement party this morning, and he celebrated by coming up with an entirely new reason for why he won't release any of his tax returns before 2010:

John Kerry ran for president. You know, his wife, who has hundreds of millions of dollars, she never released her tax returns. Somehow this wasn't an issue.

Right. It wasn't an issue. Because she was his wife. And she had that money before she married John Kerry. And John Kerry was the guy running for president. And he released two decades of tax returns. Unlike Mitt Romney.

But that's not Mitt's only explanation for his secrecy:

The Obama campaign people keep on wanting more and more and more, more things for their opposition research to pick through and make a mountain out of.

Uh, folks. Mitt Romney just admitted that the reason he's not releasing his tax returns is that he thinks releasing them would be a political gift for the Obama campaign because there's something in there that they would use against him.

The question is: What is that something? What is there in his tax returns that he doesn't want used against him? It's not that he's rich (we already know). It's not that he pays a small tax rate (we already know). It's something else. What?