Since Romney was the founder of Bain Capital, it would not be at all unusual for him to take something like a third of the partnership's "carried interest" for himself. So of the 20 percent stake that the Bain partners would keep, it is likely Romney, as the founder, would get at least a third of it -- in every deal during his 15 years at the helm. And this is where the calculus for Romney begins to get very interesting, especially if he used his IRA to invest in these buyouts.

Michele Davis, a Romney campaign aide brought on to defend his record at Bain, declined to get into specifics when asked about the IRA. "Mitt Romney has been scrupulous about observing the requirements of the tax code," she said. "As we have said many times before, Governor and Mrs. Romney's assets are managed on a blind basis. They do not control the investment of these assets, the investment decisions are made by a trustee. The IRA, like all IRAs, is tax deferred and Governor Romney will pay taxes on those funds when they withdraw the funds."

The private equity partners I spoke with laid out what they see as the most probable scenario explaining the growth of his IRA: When it came time for Romney to invest his portion of the $1 million needed (in our hypothetical example) for a Bain leveraged-buyout, instead of using money in his bank account, he used the money -- the $30,000 -- he had put in his IRA. Where once he had $30,000 in cash in his IRA, now he would have had something he valued at $30,000 but that was really his portion of the "carried interest" in the deal; if the deal worked out, the IRA could quickly be worth a lot more than $30,000.

For instance, in the hypothetical $500 million deal, with $100 million of equity, say that company did nicely and Bain sold it for $1 billion after a few years. The $400 million of debt on the company would be paid, leaving $600 million in profit for the Bain investors who put up the $100 million in equity. By the terms of the agreement between the Bain general partners and the Bain limited partners, the Bain general partners would take 20 pecent of that profit, or $120 million, leaving $480 million for the limited partners. By Romney's agreement with the other Bain guys, he would take one-third of the $120 million, or $40 million himself.

And voila, his initial $30,000 in his IRA would now be worth $40 million from one successful deal. The good news is that he could then use the $40 million in the IRA, add $30,000 each year to it, and continue to invest in one Bain Capital deal after another. In short order, his IRA could be crackling with the up to $102 million he says is in it.

The bad news for Romney is that while the contents of an IRA can compound year after year on a tax-deferred basis, money cannot be withdrawn penalty-free from the IRA until the owner of the account is 59 and a half years old. The IRA is required to be withdrawn -- and the long-deferred taxes paid -- on an annual basis starting at age 70.

"Your carry is basically buying stock for five cents that other people have paid a $1.10 for," explained one private-equity partner.

Most private-equity guys like to spend the money they get from investing in deals on toys such as vacation homes, private jets, boats, and mansions. Romney has many of these things, but he obviously had sufficient money from other sources -- inheritance, maybe? -- to pay for them, since he could not use any of the accumulated $102 million in his IRA without suffering a huge penalty. "You wouldn't put your whole investment in unless you were already loaded," one private-equity partner told me.