It's a secret.

GOP Rep. Walter Jones (R-N.C.) said Thursday that Mitt Romney's refusal to release more than two years of tax returns could hurt him in the presidential election. "I don't think this will go away," Jones told The Hill. "And if we're still talking about this in September, he's in deep trouble."

I can't see how it could go away, at this point. The longer it drags on, the more it looks like Romney really does have something to hide. That's not to say it would be a deciding factor, mind you, but it fits so well into the existing narrative of Romney being a rich, privileged jerk who doesn't think little-people rules apply to him that people are going to remember it.