Mitt Romney on campaign trail this month. (Photo: AP)

This could prove to be one heck of a required minimum distribution. The Wall Street Journal’s Mark Maremont reports that Mitt Romney has between $20.7 million and $101.6 million in his individual retirement account, due mainly to investments in Bain Capital, the private equity firm Romney helped start.

“Experts on estate planning said it is highly unusual to accumulate such a considerable sum in an IRA, an investment vehicle restricted by annual contribution limits,” Maremont writes. “Several estate-planning experts said they know of others with IRAs of more than $100 million, but they are rare. Typically, they said, that occurs when founders of companies invest in their own shares, which then take off.”

Maremont quotes Jonathan Rikoon, a lawyer at New York’s Debevoise & Plimpton LLP who advises private-equity-fund executives on estate planning. Rikoon said Romney’s reliance on a tax-deferred retirement plan for so much of his wealth “could end up costing him.” An IRA allows a small immediate tax savings, plus deferral of taxes, he explained. But he noted income from the account, when eventually withdrawn, will be taxed at the higher ordinary-income rate, not the lower capital-gains rate that might have applied if Romney had held the investments outside the fund.

“It’s probably not a slam dunk” from a tax-efficiency standpoint, Rikoon added, but it is impossible to tell without knowing the rest of Romney’s estate plan.

The revelation comes as critics and Republican opponents have pressed Romney in recent days to release his tax returns. In a further twist, ABC News is reporting that Romney “has millions of dollars of his personal wealth in investment funds set up in the Cayman Islands, a notorious Caribbean tax haven.”

A spokesperson for the Romney campaign told ABC that Romney follows all tax laws and he would pay the same in taxes regardless of where the funds are based.

As the network notes, Romney disclosed on Tuesday that he has been paying a far lower percentage in taxes than most Americans, around 15% of his annual earnings. In an increasingly contentious campaign, it has been Romney’s Republican rivals who have pressed him hardest on his tax issues.