Mitt Romney tomorrow will release his 2010 and 2011 tax returns, after which folks like me will spend hours going through the details.

And I do mean hours (plural), because these returns will be hundreds of pages long. There’s no short form when you’re invested in dozens of private investment partnerships.

Here’s what I’ll be looking for:

Overall rate: Romney last week admitted that he pays closer to a 15% tax rate than the top income rate of 35%, so this will just be a question of how much closer to 15%. My guess is just a couple percentage points away.

Bain cost basis: Romney’s retirement package with Bain Capital — a 10-year agreement that expired in 2009 — allowed for three things:

1. 1. A guaranteed percentage of all future Bain funds raised during the period, without having to actually invest in the funds. This basically means that he got to pay 15% capital gains rates on any subsequent profits, even though he didn’t originally put any capital at risk. Very common with retiring partners of private equity firms — at least those that aren’t publicly-traded — and totally legal.
2. 2. The ability to also invest into the aforementioned funds, for a larger overall percentage.
3. 3. The option to co-invest with Bain on certain large deals (i.e., write checks for select transactions). Romney would not have been able to cherry-pick deals, but instead would have agreed to participate in co-investment funds that were raised in tandem with the general funds (an option granted to most, if not all, of Bain’s investors).

My understanding is that Romney availed himself of all three options, although I won’t know the specifics until reading through the returns.   For example, Romney’s prior financial disclosures include stakes in a series of funds called BCIP Associates, which are Bain’s co-investment vehicles. By looking at Romney’s returns, we should be able to see how much of his overall rate was affected by actual capital invested (co-investments plus additional Bain fund stakes), as opposed to how much came via his retirement package (i.e., capital gains without capital put at risk).

Gubernatorial conflicts: Any actual Romney investments (#’s 2 and 3 above) made while governor of Massachusetts from 2003 to 2006 would have been made via a blind trust. But that’s little more than a smokescreen — as Romney himself once argued about Ted Kennedy’s use of such trusts — given that large Bain transactions are well-publicized. So the real question is if Romney was involved in any decisions that gave state business to a Bain portfolio company in which he had a money at risk.

Cayman Islands: A lot has been made about how Romney is invested in certain Bain Capital funds whose address is Cayman Islands instead of 200 Clarendon Street in Boston. It makes for great political soundbites, but is much ado about nothing. Almost all PE firms that raise money from overseas sources use such tax havens to protect their foreign investors from being double-taxed, and Romney’s records should show that he didn’t derive any personal gain from the Cayman mailboxes.

Bain sales: Did Bain Capital sell any companies, or stakes in any companies, in 2010 or 2011 that it didn’t publicly disclose? If so, Romney’s returns may reveal them.

Overall, I’m not expecting any bombshells from Romney’s tax returns. His reticence to release them is actually a bit confounding, particularly after publicly acknowledging the 15% figure. Maybe he was worried that South Carolina evangelicals would be turned off by his considerable tithing to the Church of Latter Day Saints. Or maybe his campaign just got caught flat-footed, not expecting Romney’s finances to become an issue during GOP primary season. Either way, we’ll have them soon. Microscopes at the ready…

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