Alex Altman makes it:

Set aside the moral or civic cases for the virtues of transparency. From a purely political perspective, Romney should dig in his heels on this position, particularly if we assume he’s correctly weighed the consequences of his options. Suppose, to pick one possibility at random, that Romney paid a very low tax rate for one or more years. Such a disclosure wouldn’t end the discussion; it would fan the flames. The Obama campaign would cut more ads casting Romney as a rapacious capitalist, except these would be buttressed by new facts in place of mere insinuations.  

Adam Sorensen counters:

Morality and civic virtue don’t have anything to do with presidential politics now? That’s nuts.

Romney should release his returns on principle alone. Full financial disclosure is important in politics because you never know what tomfoolery human beings interested in power might have gotten up to. This isn’t case-specific: Romney strikes me as an upstanding guy who’s probably never so much as jay walked in his life. He’s an eagle scout. Just a rich one with a low tax rate. But as a general rule, we’re much better off erring on the side of disclosure with our leaders. We’re handing off the nuclear football, after all. If that’s not compelling enough, there’s policy value in seeing how little he paid. Call it a teachable moment for American revenue.

Drum bets Romney won't release the returns:

At this point, he's taken such a strong stand against it that he'd look like he was caving in to the liberal media if he changed course. Standing up to the lefty establishment is a key part of his Mitt 3.0 persona, and he can't afford to damage it.

If he gets away with this, it will mark a new Palin-level of opacity with respect to the democratic process. It's creepy, arrogant and contemptuous of the good judgment of the American people.