A monkey throwing darts to pick stocks would have made a 150% profit during the decade-long run when Donald Trump’s namesake company, Trump Entertainment Resorts, lost 90 cents on the dollar, the billionaire Warren Buffett said at a Hillary Clinton campaign event Monday in Omaha, Neb.

Buffett challenged the Republican presidential nominee’s business acumen and leadership, Trump’s truthfulness about the reasons for not releasing his personal income tax return, and even the veracity of his net worth, at a rally for Democratic nominee Clinton.

Saying that the income statement Trump voluntarily provided to the Federal Election Commission last year and the tax return he must file with the Internal Revenue Service “are two different animals,” Buffett called on the Republican standard-bearer to engage with him in something of a Full Monty: with the two of them jointly presenting their tax returns to the public — and taking questions.

“Nobody’s going to arrest us. There are no rules against showing your tax returns. And just let people ask us questions about the items that are on there,” suggested Buffett, with Clinton seated beside him.

He told the cheering Nebraska crowd, “You’re only afraid if you’ve got something to be afraid about. And he’s not afraid because of the IRS. He’s afraid because of you.”

Hillary Clinton hugs Warren Buffett during a campaign rally Monday at an Omaha, Neb., high school. Reuters

Buffett’s challenge renews questions about Trump’s résumé that I and others have been asking for some time. Nearly a year ago, I pointed out that while Trump’s Miss Universe pageant had been profitable for him, it was a debacle for the host city of Doral, Fla., where he operates a resort property. In March, I presented an analysis concluding that Trump had “underperformed the real estate market by approximately $13.2 billion, or 57%,” since 1976, according to John Griffin, a finance professor at the University of Texas, who compared Trump’s stated net worth with four decades of returns on the FTSE NAREIT All Equity REITS FNER FNERXXXX, -2.19% index.

The NAREIT index, developed in the early 1970s by the Washington-based trade group the National Association of Real Estate Investment Trusts, tracks the performance of publicly traded real estate investment companies that own commercial properties, such as offices, hotels and apartments.

Starting with Trump’s professed net worth of “more than $200 million” in an interview published by the New York Times in November 1976, Griffin says the FTSE NAREIT All Equity REITS index compounded return would have returned $23.2 billon at the end of 2015, “considerably above Mr. Trump’s recent self-reported net worth of approximately $10 billion.”

Rather than rely on Trump’s word alone, Griffin also calculated the developer’s returns based on two independent assessments of his wealth: a BusinessWeek estimate that he was worth $100 million in 1978 and Forbes magazine’s analysis that puts his current fortune at about $4.5 billion.

Starting with the BusinessWeek number, the NAREIT index would have returned $8.6 billion by the end of last year, which is nearly double the Forbes estimate and consistent with “an underperformance of 48%,” Griffin says.

One caveat is that the NAREIT index doesn’t take into account living expenses and taxes. It’s not possible to estimate Trump’s real-estate tax rate, as he has not released his personal income taxes. Real estate investors pay no income tax so long as they roll any asset sale into a like-kind real estate venture.

'Have you no sense of decency?'

However, Griffin also points out that investing in a passive REIT also has a cost: At an average expense ratio of 1.03%, a $23 billion REIT position would cost the investor about $230 million a year in fees, an amount that would go a long way toward meeting living expenses.

“These numbers,” Griffin adds, “do not take into account leverage. The REIT equity funds have current leverage rations around 36%, while Trump is documented to have been levered to 69%. If one were to lever the REIT returns at Trump’s rate, then the index’s performance would be considerably higher.”

So “not only is the $13.2 billion a potential understatement of Trump’s underperformance,” Griffin concludes, “but also it seems he needed to take on a market-exceeding amount of risk in order to achieve his performance.”

Trump Hotels and Casino Resorts went public in June 1995 and quickly surpassed $1 billion in market capitalization, making Trump’s 41% stake worth about $400 million at the time. However, the company lost money every year for the next decade, during which time Trump was paid $44 million in compensation, according to Buffett. (Trump Hotels was renamed Trump Entertainment Resorts in 2004, and Trump stepped down as CEO in 2005.)

“In 1995, when he offered this company,” Buffett said, “if a monkey had thrown a dart at the stock page, the monkey, on average, would have made 150%. But the people that believed in him, that listened to his siren song, came away losing well over 90 cents on the dollar. They got back less than a dime.”

Buffett continued: “You will learn a whole lot more about Donald Trump if he produces his income-tax return. And so that’s why I’d like to make him an offer — an offer I hope he can’t refuse. ... I’ve got news for him. I’m under audit, too. I would be delighted to meet him any place, any time, between now and the election. I’ll bring my tax return. He can bring his tax return.”