Donald Trump called Club for Growth president Chris Chocola “a fool” in a new interview with ABC’s George Stephanopoulos.

Responding to Chocola’s criticisms about his support of trade protectionism, Trump told Stephanopoulos that he didn’t “know anything about the Club for Growth” but if Chocola was against tariffs on China, “he’s not a very smart man.”

“China is going to make $300 billion this year, largely because our companies can’t compete, because they manipulate their currencies,” Trump said. “Now, if he [Chocola] wants to continue to do business with a country that is taking our jobs, taking our money, and then loaning the money back to us so we pay interest to ’em. And they do it through largely a manipulation of their currency. Then he’s a fool.”

In response to Trump’s insult, Chocola told National Review Online that it “takes one to know one.”

“One thing that all economists can agree on, regardless of their political leanings left or right, from Paul Krugman and Robert Reich on the left and Art Laffer on the right, is that free trade is beneficial, creates jobs, creates economic value and economic growth and increases the standard of living,” Chocola said.

“If Donald Trump doesn’t understand that,” he added, “then he ought to maybe talk to some people who understand these issues very well and see if they’re fools.”

In the interview, Trump also continued his birther push. “Maybe I’m going to do the tax returns when Obama does his birth certificate,” he said, talking about making the financial disclosures a candidacy would require.