OMAHA, Neb. (Reuters) - Billionaire investor Warren Buffett on Monday campaigned alongside U.S. Democratic presidential candidate Hillary Clinton at a rowdy rally in his home state of Nebraska, where he challenged Republican Donald Trump to release his tax returns and questioned Trump’s business acumen.

Trump, a New York real estate developer making his first run at public office, has said he cannot release his tax returns, a ritual of U.S. presidential campaigns, until the Internal Revenue Service has completed an audit.

“Now I’ve got news for him," said Buffett, whose Berkshire Hathaway BRKa.N conglomerate is based in Omaha. "I'm under audit, too, and I would be delighted to meet him anyplace, anytime, before the election.

“I’ll bring my tax return, he can bring his tax return ... and let people ask us questions about the items that are on there,” Buffett added, saying Trump was “afraid” not of the tax-collecting IRS but of voters.

In response, Trump’s spokeswoman Hope Hicks said: “As you know, Mr. Trump is undergoing a routine audit.” She had no immediate comment when asked to respond to Buffett saying that he too was under audit but would release his tax returns.

Trump has asserted his success as a businessman qualifies him to lead the country, but Buffett, who backs Clinton in the Nov. 8 election, said Trump lost money the only time he went to the American people and asked them to invest.

He said it was in 1995 when Trump listed his Trump hotels and casino resorts on the New York Stock Exchange. He said the company lost money every year for the next decade. A monkey would have outperformed Trump’s company, Buffett said.

In 1995, “if a monkey had thrown a dart at the stock page, the monkey on average would have made 150 percent,” he said.

Buffett spoke for nearly 30 minutes to a raucous capacity crowd of roughly 3,100 people in a suburban Omaha high school with Clinton sitting at his side.

Warren Buffet speaks to U.S. Democratic presidential candidate Hillary Clinton supporters during a campaign rally in Omaha, Nebraska, December 16, 2015. REUTERS/Lane Hickenbottom

He said Trump’s “final straw” was an ABC interview broadcast on Sunday in which he criticized the Muslim parents of a decorated U.S. soldier killed by a bomb in Iraq 12 years ago.

The father Khizr Khan spoke at last week’s Democratic National Convention about their son and attacked Trump for proposing a temporary ban on Muslims entering the United States.

Trump said he was “viciously” attacked by Khan, a naturalized U.S. citizen, when the father publicly doubted Trump had read the U.S. Constitution. Khan said that Trump had “sacrificed nothing,” prompting Trump in his ABC interview to say, “I think I’ve made a lot of sacrifices.”

Buffett on Monday bluntly contradicted Trump.

“No member of the Buffett family has gone to Iraq or Afghanistan. No member of the Trump family has gone to Iraq or Afghanistan,” Buffett said. “We’ve both done extremely well during this period and our families haven’t sacrificed anything.”

In his remarks Buffett announced the launch of a get-out-the vote effort, pledging to take at least 10 people to the polls who would otherwise have difficulty getting there. Buffett said he was backing a website, Drive2Vote, that would coordinate transportation to cast votes and that he had reserved a trolley that seats 32 people for the same purpose.

“I’m going to be on it all day. I’m going to do selfies, whatever it takes,” Buffett said.

Buffett said his goal is to generate the highest voter turnout in the congressional district that includes Omaha of any in the country. Nebraska is one of two U.S. states that award electoral votes in presidential elections by congressional district.

Clinton responded to Buffett’s pledge with a promise of her own, if his turnout goal is met.

“Warren and I will dance in the streets of Omaha together! Maybe if we’re really lucky he’ll wear his Elvis costume again!” she said.