Dan Shaviro (NYU), Why Won't Romney Release His 2009 Tax Return?:

Increasingly, everyone (including Republicans such as George Will, Matthew Dowd, and William Kristol) agrees that the reason for Romney's reluctance to release any pre-2010 tax return might be that what it would show is worse than all the heat he is taking for non-disclosure.

But what could that be?  Well, I certainly don't know either, but here are some salient points:

1) We know from the 2010 tax return, in which he had a net capital loss carryforward from 2009, that he zeroed out his net capital gains – including from carried interest Bain income - in 2009.

2) 2009 was the last year in which he received certain Bain payments as the playout of his "retroactive retirement."

3) It's been hard to understand what benefit he thought he was getting from the Swiss bank account, and there was an IRS amnesty program in 2009 for fraudulent nondisclosure of offshore income.  If he had to come clean in 2009, this might be embarrassing, especially given that there was an iron fist inside the IRS leniency offer (i.e., if you held out, they might get you without any amnesty).

From the first two items above, it may be a reasonable guess that Romney had a lot more gross income in 2009 than 2010 and 2011, yet paid less tax or even zero tax.

• The New Yorker, Why Won’t Romney Release More Tax Returns?
• Wall Street Journal, Romney Rejects Calls on Tax Returns
• Washington Post, New Obama Attack: Maybe Romney Didn't Pay Taxes