WASHINGTON – President Trump's lunch on Tuesday with Rex Tillerson might have been a little tense given the president's latest volley in his feud with the secretary of State.

A week after reports that Tillerson once called Trump "a moron," the president told Forbes magazine:

"I think it's fake news, but if he did that, I guess we'll have to compare IQ tests. And I can tell you who is going to win."

If the president is serious, then he has at least one willing participant: Mensa, the high IQ society.

"American Mensa would be happy to hold a testing session for President Trump and Secretary Tillerson,” Charles Brown, the group’s communications director, told The Hill.

Trump's challenge to compare IQ tests won't tap down speculation that Tillerson may depart the Trump administration. And the president, in the interview, doubled down on recent criticism of his own secretary of State's approach to high-stakes national security challenges.

Just one day after Tillerson first acknowledged the administration is seeking a dialogue with North Korea about its missile and nuclear tests, Trump on Oct. 1 tweeted that his top diplomat "is wasting his time trying to negotiate with Little Rocket Man." The president was using his nickname for North Korean leader Kim Jong Un.

"He was wasting his time," Trump repeated in the Forbes interview, and defended his public Twitter rebuke of Tillerson. "I'm not undermining ... I think I'm actually strengthening authority," he said.

As Forbes put it: "It's hard to see whose authority he's strengthening, other than his own."

Last week, Tillerson took the unusual step of making a televised statement to refute an NBC News report that he had considered resigning his post, and was so frustrated with Trump that he called him a "moron" during a July 20 meeting at the Pentagon. Trump later said he had total confidence in his secretary of State.

Defense Secretary James Mattis also attended the White House lunch; maybe he helped break the ice.

White House spokeswoman Sarah Sanders, meanwhile, downplayed the president's IQ comment, saying "he made a joke, nothing more than that."

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