WASHINGTON, Aug. 2 (Xinhua) -- Legendary investor Warren Buffett Monday challenged Republican presidential nominee Donald Trump over his refusal to release tax returns as well as his claim that he's "made a lot of sacrifices."

At a rally with Democratic presidential nominee Hillary Clinton in his hometown Omaha, Nebraska, the second richest man in the United States said he would meet Trump "any place, any time between now and election" to look into each other's tax returns, suggesting Trump has something to hide.

"Now I've got news for him, I'm under audit too," Buffett said, rebuking Trump's claim that he can't release tax returns because he is being audited.

"There are no rules against showing your tax returns and just let people ask us questions about the items that are on there," said Buffett.

"You're only afraid if you've got something to be afraid of," he told the crowd. "He's not afraid of the IRS (Internal Revenue Service, the U.S. government agency responsible for tax collection and tax law enforcement). He's afraid because of you."

"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."

"How in the world can you stand up to a couple of parents who lost a son and talk about sacrificing because you were building a bunch of buildings?" Buffett said, criticizing Trump for his attacks on the parents of a Muslim-American soldier killed in Iraq 12 years ago.

"Have you no decency, sir?" Buffett asked.

He then said he is launching a website called drive2vote.org, where he has reserved a trolley to drive people to vote.

"It seats 32. I'm going to be on it all day. I'm going to do selfies, whatever it takes," Buffett said. "My goal is to have the turnout here be the highest percentage of potential voters of any district in the country."

Buffett's strong opposition comes after a speech made by former New York mayor Michael Bloomberg, calling for unity of independent voters against Trump at the Democratic National Convention last week. The two billionaires questioned Trump's success as a businessman since the Republican wild-card candidate has always argued that he is able to run America as successfully as he runs his own business.