OMAHA — The investor and philanthropist Warren Buffett unleashed a withering attack on Donald J. Trump on Monday for refusing to release his tax returns, asserting he had something to hide, and for misleading voters about his success as a businessman and ability to improve the American economy.

Mr. Buffett, known to investors as the Sage of Omaha, said a monkey throwing darts at the stock pages in 1995, when Mr. Trump first offered stock in his Atlantic City hotels, would have come out far ahead of anyone who listened to Mr. Trump’s “siren song” and invested in his company that lost money year over year.

The attack from perhaps the nation’s most revered investor undercut a core argument of Mr. Trump’s presidential candidacy: that his success as a businessman qualified him to run the country despite his lack of political experience.

Mr. Buffett appeared at a rally with Hillary Clinton, the Democratic nominee, in his hometown, Omaha. Taking on Mr. Trump, he pledged to personally transport 10 Nebraskans to the polls on Election Day on behalf of Mrs. Clinton and urged others to do the same.