AP Photo/Alex Brandon
Donald Trump.
Trump shot back on Thursday, calling Clapper a liar and implying that he is a hypocrite.
"James Clapper, who famously got caught lying to Congress, is now an authority on Donald Trump," Trump tweeted on Thursday morning. "Will he show you his beautiful letter to me?"
In the first part of his tweet, Trump was referring to Clapper's 2013 testimony before Congress in which he denied that intelligence officials "wittingly" collected data on millions of Americans. Many of Clapper's critics accused him of perjury following public revelation of the National Security Agency's mass surveillance programs.
It is unclear what letter Trump was referring to.
The president's tweet was likely provoked by Clapper's comments on CNN following Trump's campaign rally in Arizona on Tuesday night, which the veteran intelligence official called "downright scary and disturbing."
"I really question his ability to - his fitness to be - in this office, and I also am beginning to wonder about his motivation for it - maybe he is looking for a way out," Clapper said.
Clapper said the president exhibits a "complete intellectual, moral, and ethical void."
"How much longer does the country have, to borrow a phrase, endure this nightmare?" he asked.
When CNN host Don Lemon asked if Trump is a threat to national security, Clapper responded that he "certainly could be" and pointed to the president's escalating rhetoric concerning North Korea.
"In a fit of pique he decides to do something about Kim Jong Un, there's actually very little to stop him," Clapper said, referring to the North Korean dictator. "The whole system is built to ensure rapid response if necessary. So there's very little in the way of controls over exercising a nuclear option, which is pretty damn scary."