Donald Trump has hit out at a former staff member who's written a damning tell-all book about his time in the White House, calling Cliff Sims a “gofer” he hardly knew—all while the author was live on CNN.

Revelations from Sim's book, Team of Vipers: My 500 Extraordinary Days in the Trump White House, have dripped out through the media over the past couple of weeks. They include Trump drafting naughty and nice lists of staff, the president's White House tours of where President Bill Clinton and MonicaLewinsky had sex, and a quote from Stephen Miller saying that he “would be happy if not a single refugee foot ever again touched America’s soil.”

Trump wrote Tuesday morning: “A low level staffer that I hardly knew named Cliff Sims wrote yet another boring book based on made up stories and fiction. He pretended to be an insider when in fact he was nothing more than a gofer. He signed a non-disclosure agreement. He is a mess!”

The executive director of the Donald Trump for President campaign, Michael Glassner, claimed shortly afterwards that Trump is preparing a lawsuit against Sims for an alleged breach of that NDA.

As Trump's tweet went out, Sims was appearing on CNN's New Day and, clearly overjoyed at the hint that the president was tuning in, the network dragged the tweet up on the big screen and read it aloud to Sims.

With a somewhat strained smile on his face, the author responded: “Nice... there it is. Look, I knew that was a possibility when I wrote that book and you know what I said. My identity is not wrapped up in being a Trump staffer. My identity is wrapped up in who I am in my faith and those are things that matter to me. I know who Jesus says I am. It doesn't matter to me what Donald Trump or anyone else says I am.”

Asked if he was bothered at being called a “gofer,” he insisted: “It doesn't matter to me one bit.”

Host Alisyn Camerota finished off the interview by saying, “Thank you, Mr. President, for watching our segment and our show today.”

Sim's insider memoir Team of Vipers was released Tuesday.