The race for the White House has turned into a battle between billionaires.

Berkshire Hathaway CEO Warren Buffett, Dallas Mavericks owner Mark Cuban, and former New York City Mayor Michael Bloomberg have all recently come out against Republican presidential nominee Donald Trump.

Buffett joined Democratic presidential nominee Hillary Clinton on the campaign trail Monday, and used his introductory speech to take aim at Trump and his business record.

Buffett also challenged Trump to release his taxes to the American people. "Nobody's going to arrest us, there are no rules against showing your tax returns," the Oracle of Omaha said.