As if there weren’t enough questions involving taxes swirling around Mitt Romney, his offshore accounts make his tax returns that much more important.

The Romney campaign has pushed back against the reports this morning, arguing that the Republican is not using his offshore strategy to avoid U.S. taxes. The Wall Street Journal reports the campaign’s line is dubious.

Tax experts said some of the offshore holdings are likely intended to help Mr. Romney avoid paying an obscure but hefty tax of as much as 35% on some of those investments, held in a tax-deferred retirement account.

As The Wall Street Journal reported in Thursday’s paper, many of Mr. Romney’s offshore investments are held through his individual retirement account, which has grown to between $20.7 million and $101.6 million. IRAs are tax-deferred accounts, in which earnings accrue tax-free until the money is withdrawn during retirement.

Mr. Romney’s IRA has grown so large, it appears, due to investments in various vehicles managed by Bain Capital, the investment fund he helped found in 1984. His latest financial disclosure report, filed in August, shows that many of the IRAs assets are in Bain-affiliated entities located offshore, including one in the Cayman Islands that the report listed as having a value of between $5 million and $25 million.

The WSJ report added that if those funds were in the U.S., Romney would likely have been subjected to an obscure levy called the “unrelated business income tax.” By going offshore, Romney is avoiding the tax bill.