Warren Buffett is one of the richest people in the world (third on the list, according to Forbes  most recent ranking, and he is backing Hillary Clinton for President of the United States. Buffett appeared with Clinton at a campaign event on Monday, and he took aim at Donald Trump.Buffett, campaigning on behalf of Democratic presidential nominee Hillary Clinton, challenged Trump to meet him  any place, any time  with a copy of his tax returns. Buffett said he would bring his returns and the two of them would answer questions from the public. He dismissed Trump s contention that he couldn t release them because he s being audited. Buffett said his return is being examined by the Internal Revenue Service as well. You re only afraid if you ve got something to be afraid about,  Buffett told a cheering crowd in Omaha.  He s not afraid because of the IRS, he s afraid because of you. While it has been impossible to verify Trump s claims to wealth, investor Buffett   often described as the  oracle of Omaha,  has a well documented fortune that is estimated to be $60.8 billion dollars. Also unlike Trump Buffett is known for living a relatively humble life style for someone with his sort of rent. He lives in a relatively modest home in Nebraska (no gold plating) and traded in his 2006 car for a 2014 Cadillac.Buffett is the third well-known billionaire to come out in support of Clinton and bash Trump. Former New York City mayor Michael Bloomberg endorsed Clinton at the Democratic convention, while Dallas Mavericks owner Mark Cuban praised Clinton at a Pennsylvania rally while calling Trump a  jagoff. Trump has claimed to be worth  $10 billion dollars  but has offered no documentation to prove his net worth or show what his tax payments have been. Previous testimony from Trump has shown him wildly changing on his net worth, and he sued an author who documented an investigation showing Trump to be worth several million but not among the world s billionaires.Featured image via YouTube