Warren Buffett is out to smash the cornerstone of Donald Trump's campaign--that the Republican presidential candidate's business acumen qualifies him to run the country. 

In Nebraska on Monday, the beloved investor and philanthropist criticized Trump for not releasing his income tax returns, saying Trump must have something to hide.

Throughout the election cycle, Trump has had a few reasons for not releasing his returns. Buffett had a rebuttal for them all. Trump claims he filed financial statements to the Election Commission, but Buffett is quick to point out that an income tax return is a much different beast. 

"You will learn a lot more about Donald Trump if he produces those income tax returns," Buffett told the crowd in his hometown of Omaha. 

Trump also said he can't release his income tax returns because he is currently under audit by the IRS. Well, Buffett has an offer for him: "I'm under audit too," he says, and challenged Trump to produce his own tax returns before the election, promising he'd  stand with Trump as both men answer questions about their records.

"He's not afraid because of the IRS," Buffett says to the crowd. "He's afraid because of you."

The reliably cautious Buffett has been hesitant to make strong statements on Trump, and instead sticks to praising Clinton, whom he's endorsed. But on Monday he said, "The final straw came this weekend," when Trump appeared on This Week With George Stephanopoulos and discussed how he's made sacrifices for his country by building and providing jobs. (The subject of sacrifice has been very much in the news thanks to Trump's ongoing feud with Khzir Khan, the father of a Muslim-American captain killed in Iraq, who at the Democratic National Convention famously called out Trump: "You have sacrificed nothing!") 

Buffett said, "How in the world can you stand up to a couple of parents who have lost a son and talk about sacrificing because you were building a bunch of buildings?" 

Buffett also blasted Trump for leading the American people to believe he is a competent businessman. Between 1995 and 2005, Buffett says, Trump Hotels, trading under ticker symbol DJT, lost money every year during a time when a random selection of stocks (or, as Buffett memorably put it, "a monkey throwing a dart") would've earned 150 percent. 

Buffett's tart comments directed at Trump came two days after another billionaire Clinton endorser, Mark Cuban, ridiculed Trump at a Clinton rally in Western Pennsylvania. In his remarks, Cuban referred to Trump--among other things--with one memorable Pittsburgh putdown: "jagoff."