Weighing in on a controversy that has been looming for weeks, billionaire investor Warren Buffett on CNBC this morning called the Employee Free Choice Act a “mistake.”

Known to opponents as “card check,” the proposal would give workers the option to bypass secret elections as they decide whether to form a union. Instead of an election run by the National Labor Relations Board, workers could fill out cards saying whether they support or oppose a union.

Though the card check bill has not advanced, both sides are itching for a fight later this year.

“I think the secret ballot’s pretty important in the country. I’m against card check to make a perfectly flat statement,” said Buffett. He later added that “the people who are in unions have not been well treated by the tax code that we’ve had.”

Take Action: Tell Your Representative to Oppose the Employee Free Choice Act!