If the tax return issue actually becomes a big deal to the general public, then I think he'll flip flop on it pretty readily just to get the conversation back where he wants it.
When you add in that his tax plan would cut that even lower it's very bad political optics.
He paid 13% in a year that he absolutely KNEW he would have to release the data when he was planning on running for president.
As one of his few concrete campaign proposals is to lower taxes paid by the rich, this would obviously be damaging.
The less likely but more fun theory is that he took place in a recent IRS amnesty offer to repatriate money held in Switzarland, after the US got the Swiss to agree to various reporting rules.
That sounds like an argument he should release the returns.
If everybody already has a general idea what's in them and the public doesn't care, why allow the Obama campaign to keep hitting him with 'Mitt Romney keeps his money overseas and won't release his tax returns' ads?
Most people probably just assume that the returns will show that he's a rich guy who pays a relatively low tax rate and have already baked that into the cake of their perceptions of the guy.
It's going to be in a lot of campaign ads in September and October, and it's unlikely to help if he releases them at that point.
Even if everything was technically legal (such as somehow declaring his dressage horse as a business expense), it seems like it would be hard to convince the average American that this is the person that we'd want in charge.
If it does somehow come out that he paid single digit percentages, or less, is that game over for him or will it somehow be forgiven?
Also: when are we supposed to see last years return?