A day after de facto Republican presidential nominee Donald Trump said "there's nothing to learn" from making his tax returns public before this November's elections, the billionaire is taking heat from the party's 2012 nominee over that stance.

"It is disqualifying for a modern-day presidential nominee to refuse to release tax returns to the voters," said Mitt Romney in a post on Facebook.

Trump insists he won't release his returns until the Internal Revenue Service has finished auditing them. An NPR fact check from February found that there was no law that prohibited the release of tax returns that were under audit.

There is a curious echo to Romney's accusation of a "bombshell" in Trump's tax returns. Romney, the former CEO of a private equity firm, came under similar scrutiny in 2012 when he released only two years of tax returns in 2012 — and after coming under considerable pressure from the Obama campaign and some Republicans.

Trump's decision to withhold the tax returns would break a nearly 40-year tradition for presidents and major presidential candidates, according to the Tax History Project.

"There is only one logical explanation for Mr. Trump's refusal to release his returns: There is a bombshell in them," Romney said. "Given Mr. Trump's equanimity with other flaws in his history, we can only assume it's a bombshell of unusual size."

The real estate billionaire's actual wealth has been a subject of debate since he entered the race last summer. Trump claimed his net worth was $10 billion, although Forbes magazine estimated it at $4 billion. Trump also released a 92-page financial disclosure form that didn't shed light on his net worth.

In addition to giving a sense of Trump's annual income and wealth, tax returns would also reveal Trump's charitable contributions and the tax deductions and credits from which he benefits.

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