Since he won't release his tax returns, and does admit they'd be politically damaging, all we can do is guess. This diary is one in a series of speculative diaries wondering what could possibly be worth the price he is paying by remaining secretive.
Today's idea: Romney is good at making money ... but most Americans won't understand that from looking at his taxes. Romney created partnerships like hog farms create stench. I guess that a lot of those partnerships didn't make any money, or didn't make very much. There were only a few which really paid off; IIRC, Bain Capital made almost half its profit off of 10 deals, 4 of which resulted in a looted company going bankrupt.
So an examination of Mitt's taxes from back when he was working as an investor rather than as a casher of checks from a trust will show a man with, perhaps, a batting average of about .100. Now, because of leverage and so on that's actually not a bad batting average; but it won't look good to low-info voters.
It will also strike at GOP economics a person who can afford to lose their shirt 9 times so the 10th time can pay off 200 shirts is a person who didn't start out with nothing. That is, a guy starting his own business is taking a much greater risk than a millionaire investing in a startup. It will also show that Romney doesn't necessarily know a thing about creating jobs.
Other diaries in the series.
Partnerships with bad people
How he got $100M in his IRA
He didn't tithe
Nothing, he's just pathologically secretive
Hiding stuff from Ann Romney
Rich people write-offs
Olympics corruptions
Medical expenses
Political contributions
Foreign investments
Church contributions exceed taxes