It's not clear if voters really care about the fact that Mitt Romney hasn't revealed more than one year of taxes, but the talk surrounding this issue is so loud that it's a "win" for Obama because it's a topic other than the economy and jobs.

A growing number of Republicans think that obviously Romney should release his taxes. But only he knows what's in his returns, and he does not think it's so obvious.

So everyone has their theories.

One easy possibility is that it's just a matter of drawing more attention to the fact that he is really rich and his tax rate isn't all that high. But this doesn't make sense since this would only confirm what everyone really knows.

Joshua Green at Bloomberg has what is the most logical theory so far... which is that thanks to the economic collapse of 2008, and the massive losses he probably suffered, Romney might not have paid any taxes in 2009, thanks to tax-loss carryforwards.

As a member of the ultra-rich, Romney probably wasn’t spared major losses. And it’s possible that he suffered a large enough capital loss that, carried forward and coupled with his various offshore tax havens, he wound up paying no U.S. federal taxes at all in 2009. If true, this would be politically deadly for him. Even assuming that his return was thoroughly clean and legal — a safe assumption, it seems to me — the fallout would dwarf the controversy that attended the news that Romney had paid a tax rate of only 14 percent in 2010 and estimated he’d pay a similar rate in 2011.

We're not sure whether Green is right that it would be politically "deadly" though we could certainly see why Romney wouldn't want this known, if it were true.

Incidentally, another more exotic possibility that had come to mind is that somehow Mitt Romney made a windfall profit in 2008-2009, perhaps by being part of a fund that was short housing or bet on the bank bailouts. That's 100% speculation, but it would also be the kind of thing that would be deadly (probably more deadly than $0 taxes). It also would have been somewhat unplannable.

A problem with the tax loss carryforwards argument is that if Romney knew he'd be running in 2009 (and he probably did) then you'd think he'd have gone out of his way not to do anything that might look bad, even if 100% by the book.

Going back to the no tax issue, former political strategist and now media guy Peter Feld notes that the latest Obama TV ad specifically raises the possibility that Romney paid $0 in taxes. So in the absence of counter-evidence it seems likely that this is the theory that will become the most popular.