Mitt and Ann Romney are deluding themselves if they believe that calls for the presumptive Republican presidential nominee to release more of his income tax returns are simply a campaign instigated by Barack Obama’s supporters. Would that partisanship is sparking the demands for additional disclosure. The Romneys must know in their hearts that there is more to it.

Most Americans don’t begrudge Mitt Romney his wealth, estimated in the neighborhood of $250 million. His entrepreneurship is an American success story.

But voters also want to know why this fantastically rich seeker of the presidency is being so secretive about his tax payments and how he made his money.

Does he have something to hide?

If everything in his tax returns is above reproach, why won’t Romney follow the bipartisan tradition established by the presidential campaign of his father, George Romney, in 1968, and release more of them?

It’s not enough for Romney to say he’s paid all taxes that are “legally required.” A person who wants to be president should also be able to say, and to demonstrate, that no ethical lines have been crossed.

Romney has offshore accounts. Voters are within their rights to ask why this man who wants to be president would divert income from U.S. financial institutions to foreign tax havens.

These are not questions raised solely by the Obama camp.

Consider some points raised by tax experts in a CNN piece last month on Romney’s lack of disclosure. Edward D. Kleinbard, a professor at the University of Southern California’s Gould School of Law and former chief of staff of the congressional Joint Committee on Taxation, and Peter C. Canellos, former chair of the New York State Bar Association Tax Section, asked several good questions.

Why would Romney have a Swiss bank account? “Most presidential candidates don’t think it appropriate to bet that the U.S. dollar will lose value by speculating in Swiss Francs, which is basically the rationale offered by the trustee of Romney’s ‘blind’ trust for opening this account,” they wrote. And “you don’t need a Swiss bank account” to speculate in foreign currencies, they note.

Then they focused on the tax-compliance questions the Swiss account raises. “The account seems to have been closed early in 2010, but was the income in fact reported on earlier tax returns?” they asked. And did the Romneys file, on time, the necessary disclosure forms to the Treasury?

Then there is Romney’s sizable IRA.

“Even under the most generous assumptions,” wrote Kleinbard and Canellos, “Romney would have been restricted to annual contributions of $30,000 while he worked at Bain. How does this grow to $100 million?”

Plausible explanations exist, they said, including that “a truly mighty oak sprang up virtually overnight from relatively tiny annual acorns because of the unprecedented prescience of every one of Romney’s investment choices.” But it’s also possible, they said, that Romney may have “stuffed far more into his retirement plans each year than the maximum allowed by law by claiming that the stock of the Bain company deals that the retirement plan acquired had only a nominal value.”

Of course, we don’t know without seeing Romney’s tax paperwork.

Kleinbard and Canellos said the vast amounts in Romney’s family trusts raise a parallel question: “Did Romney report and pay gift tax on the funding of these trusts,” or might he have claimed “unreasonable valuations” that “would have exposed him to serious penalties if all the facts were known?”

The “complexity of Romney’s one publicly released tax return, with all its foreign accounts, trusts, corporations and partnerships, leaves even experts (including us) scratching their heads. Disclosure of multiple years’ tax returns is part of the answer here, but in this case it isn’t sufficient. Romney’s financial affairs are so arcane, so opaque and so tied up in his continuing income from Bain Capital that more is needed, including an explanation of the $100 million IRA.”

Next comes Romney’s low effective tax rate: 13.9 percent in 2010. (Recall that Romney said last week that over the past decade, he “never paid less than 13 percent.”)

The rate is probably low, the experts suggested, because the Romneys’ income comes from “carried interest,” which they called “the jargon used by the private equity industry for compensation received for managing other people’s money.”

“The vast majority of tax scholars and policy experts agree that awarding a super-low tax rate to this one form of labor income is completely unjustified as a policy matter,” they concluded.

So again, how did Mitt Romney make his money? What has he done with it? Why the offshore accounts?

Romney should come clean in Tampa with the Republicans who must carry his water.

Romney also should be open and transparent with the American electorate. They deserve to know his full, true story.