President Donald Trump on Tuesday morning attacked Cliff Sims over his new tell-all book, calling him a "low level" White House staffer who wrote a "boring book based on made up stories and fiction."
Soon after, the chief operating officer of Trump's re-election campaign, Michael Glassner, tweeted that the campaign was "preparing to file suit against Cliff Sims for violating" a nondisclosure agreement he had signed.
Amid a Tuesday morning media tour, Sims responded to Trump's tweet in real time on CNN's "New Day."
"Nice," Sims said as the president's tweet was read aloud. "There it is."
Sims added that it doesn't matter to him "what Donald Trump or anyone else says that I am."
Released Tuesday, the book, "Team of Vipers: My 500 Extraordinary Days in the Trump White House," provided a firsthand account of West Wing chaos and infighting among the president's upper echelon of staffers. On Monday, Politico reported Trump was "very pissed off" about Sims' account.
"What I thought was missing was a firsthand account of the truth about what it’s like to work there," Sims, who also worked for Trump's campaign, told CNN. "I was a viper, too."
The book portrayed some staffers, such as White House counselor Kellyanne Conway and senior adviser Stephen Miller, in an unfavorable light, saying Conway routinely leaked to the news media and Miller spoke dismissively about refugees.
In another episode in the book, Sims writes that after Trump viciously attacked MSNBC "Morning Joe" host Mika Brzezinski in a June, 2017, tweet, the president instructed then-White House press secretary Sean Spicer, "Don’t you dare say I watch that show."
"Who doesn’t like to hate-watch 'Morning Joe?'" Sims joked in an interview with the MSNBC hosts on Tuesday.
Reacting to the president's Tuesday tweet, Conway's husband, George Conway, an attorney and frequent Trump critic, wrote on Twitter that the president's "stupidity knows no bounds."
It's unclear whether the nondisclosure agreement Sims apparently signed is enforceable. Sims did not immediately respond to a request for comment from NBC News.
Last year, the campaign filed an arbitration claim against former White House aide Omarosa Manigault Newman in response to her tell-all book.
The Washington Post, citing a person who had signed a nondisclosure agreement, reported last year that the agreements were meant to extend beyond Trump's time in office.
Jessica Levinson, a Loyola Law School professor, wrote at the time that the Trump administration did not seem to understand that it could not bar government employees from discussing their time in public service.
"Of course, Trump can attempt to bully or cajole his employees into signing any agreement he wants," Levinson wrote. "But are those agreements enforceable? It seems unlikely. There's a strong case to be made that NDAs signed by White House employees violate the First Amendment and also public policy, making it doubtful that a court would agree to enforce them."