Look at the excuses Mitt has offered up:
Democrats are mean.
His tax returns are politically damaging.
It's none of "you people's" business.
He's not a business, even though he insists "corporations are people, my friend."
His stonewalling has led to significant drops in the polls.
His coments, reports about his business dealings and his stonewalling guarantees that his tax returns are going to be thoroughly inspected. If he doesn't release them soon, he'll continue to get hammered.
It's a lose-lose for Mitt.
Which brings to mind this claim by Abby Huntsman, Jon Huntsman's daughter:
In fact, according to people close to the situation, Romney would drop out of the presidential race before ever releasing further tax returns. http://www.huffingtonpost.com/abby-huntsman/mitt-romney-tax-returns_b_1675711.html whats romney hiding