If Mitt Romney is looking for a loophole to close to help pay for his tax plan, he might want to look at one that helped him build his own retirement savings. His freakishly massive retirement savings.

For now, the Internal Revenue Service is looking into the kinds of tax schemes that reportedly have helped Romney build what is certainly one of the world's largest individual retirement accounts, the Wall Street Journal reports.

Laura Saunders at the WSJ's Total Return blog pointed out Wednesday that a Treasury official recently wrote a letter to Congress saying the IRS has for some time been "diligently" investigating the sorts of schemes that reportedly boosted Romney's IRA.

Here's the background: One of the many noteworthy things about Romney's steroidally enhanced wealth is the unusually large size of his IRA. According to Romney's financial disclosures, his IRA is valued at between $20.7 million and $101.6 million. The Washington Post recently pegged it at about $87 million.

Why is this unusual? Because, at the time Romney was building his IRA, people were only allowed to contribute $30,000 to them per year, at most. As William Cohan noted recently, had Romney contributed that maximum every year from 1984 to 1999 -- the years Romney was, on paper at least, either employed by Bain & Co. or running the spinoff private-equity firm Bain Capital -- then the most he could have contributed would be $450,000.

You may now take a moment to consider the vast difference between $450,000 and $87 million.

What accounts for this gap? The WSJ reported earlier this year that Romney's IRA likely swelled to that size because of an awesome Bain retirement plan that let executives buy stock in companies Bain owned at super-cheap prices and then shove them into IRAs. When those stock prices took off, then so did the value of those IRAs. Badda bing, badda boom, tax-free investment.

The one big flaw in the plan, as the WSJ pointed out, is that gains in an IRA are taxed like regular income, when you withdraw them -- 35 percent, in Romney's case -- instead of at the capital-gains rate of 15 percent.

"Who wants to have $100 million in an IRA?" a Romney campaign official asked the WSJ.

The follow-up question is: Who wouldn't want that?

Anyway, this comes at a time of high interest in all things having to do with Romney's taxes. He has refused to release more than a couple of years' worth of returns. The returns he has released have raised still more questions about how he has managed to keep his tax bill low -- paying a 14 percent rate last year. That is, if you don't count all those millions in his IRA.

Democrats in Congress say the Bain approach to IRA building is taking advantage of a loophole that they want tightened. They say Bain executives and anybody else using this approach have been cheating by undervaluing the investments they put in their IRA, to get around contribution limits. The scheme is legal, but only if investments are priced at fair market value when they go into the IRA.

Now, as the WSJ reports, the IRS is involved. It warned in late 2003 that it would be on the lookout for schemes to put under-valued assets into IRAs, but apparently people haven't exactly been paying attention to that warning. Last year, according to the Treasury official's letter, the agency convened a working group to study the problem.

"The Treasury Department and the IRS are currently working to obtain a reliable estimate of the number of audits that specifically involve IRA asset valuation issues and an estimate of the size of associate tax compliance problems," the official's letter said.

There is no indication that Romney's IRA is part of any investigation. His retirement savings are probably safe.

In any event, the IRS working group is still in the "early stages" of its review, according to the Treasury official's letter. Any changes could take a while.

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