One day after James Clapper questioned the president's fitness for office, President Trump lashed out at the former director of national intelligence, and suggested that he wrote him a "beautiful letter."

"James Clapper, who famously got caught lying to Congress, is now an authority on Donald Trump," the president said Thursday on Twitter. "Will he show you his beautiful letter to me?"

Clapper, who served as the intel chief for most of the Obama administration, called the president's Phoenix rally speech "extremely disturbing" and questioned the sitting president's ability to serve.

"I really question his ability to be — his fitness to be — in this office," Clapper said. "And I also am beginning to wonder about his motivation for it." Clapper, who left position at the helm of the U.S. intelligence community at the end of the Obama administration, also said he was concerned about the president's access to nuclear codes if he has "a fit of pique" over North Korea's Kim Jong Un.

Some lawmakers have charged that he misled Congress back in 2013, when Clapper replied to a question about whether the National Security Agency collected data on millions of Americans. Clapper said, "No, sir ... not wittingly."

Clapper has said he misunderstood the question and misspoke, insisting that he did not lie.

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