"Now, I've got news for him. I'm under audit too," Buffett said. "There are no rules against showing your tax returns and just let people ask us questions about the items that are on there."
Buffett suggested that Trump is worried about scrutiny of his tax returns — but not by the government.
"You're only afraid if you've got something to be afraid about," he told to the crowd. "He's not afraid of the IRS. He's afraid because of you."
Buffett also challenged Trump on his claim that he has "made a lot of sacrifices."
"Donald Trump and I haven't sacrificed anything," Buffett said. "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, the second-richest man in the country, closed out his introduction of Clinton with an offer to voters. Stressing Nebraska's importance come Nov. 8, he said that he is launching a website, Drive2Vote.org, to coordinate transportation to the polls and that he has reserved a trolley to take people to vote.
"It seats 32. I'm going to be on it all day. I'm going to do selfies, whatever it takes," he said. "My goal is to have the turnout here be the highest percentage of potential voters of any district in the country."
Clinton upped the ante on that goal, making a promise to return to the district if it has the highest voter turnout rate.
"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," she said. "And maybe if we're really lucky, he'll wear his Elvis costume again."