OMAHA, Nebraska -- Billionaire investor Warren Buffett made Donald Trump an offer on Monday: to meet "any place, any time" and show their tax returns to the public.

"I will meet him in Omaha or Mar a Lago, or he can pick the place," Buffett said, speaking at a rally for Hillary Clinton in Omaha. "Any time between now and the election. I'll bring my return, he'll bring his return. We're both under audit, and believe me nobody's going to stop us from talking about what's on these returns."

He told the crowd: "Send the word to him, if you will."

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Buffett introduced Clinton at the rally, an effort by the Clinton campaign to show the business community's support for the Democratic nominee. In his remarks, he described Clinton as someone who cares about the widening gap between the haves and have-nots.

"It won't cure itself," Buffett said. "It's going to take Hillary Clinton. It's going to take a mandate from the people in their election."

Buffett, who also campaigned with Clinton during the primary, is the third major business figure to publicly show support for the Democratic nominee in the last week. Over the weekend, Clinton picked up another billionaire's boost when Dallas Mavericks owner Mark Cuban endorsed her in Pittsburgh, opining, "Leadership is not yelling and screaming and intimidating," and he praised Clinton as "a true leader." At the Democratic convention, yet another billionaire, former New York City Mayor Michael Bloomberg addressed the delegates and announced his endorsement of Clinton, too, calling Trump "a risky, reckless, and radical choice."

Warren Buffett speaks at a Hillary Clinton campaign event in Omaha, Nebraska, on Monday, August 1, 2016. CBS News

Nebraska is a state that hasn't voted Democratic since 1964. But unlike most of the rest of the states, Nebraska divides its electoral votes (Maine is the other exception). The popular vote winner gets two of the five votes, and the remaining three votes are assigned individually to the winner of each of the three congressional districts.

Omaha and its suburbs make up the second congressional district, which is more liberal and more diverse than much of the state. President Barack Obama won the district narrowly in 2008 against John McCain. With their eye on a win early, the Clinton campaign went on the air with television ads in the Omaha media market at the end of June as part of a major buy in battleground states across the country.

"We live in a very special place," Buffett said. He cited an analysis by ABC News that showed Clinton and Trump in a tie, 269 electoral votes to 269. "I am looking at the people that can change that 269 to 270."Buffett offered to help take Nebraskans to the polls on November 8to make sure that more potential voters in his congressional district turn out than in "any district in the country."

"I'm going to do selfies, whatever it takes," he said.

In her remarks, Clinton upped the ante.

"If this congressional district has the highest percentage of votes in any congressional district and of course, if I win, here's what we'll do," Clinton said. "I will, shortly after I become president, sometime as soon as I can arrange it, come back here and Warren and I will dance in the streets of Omaha together!"