Mr. Nunberg soon reversed course, telling the Journal that he spent Tuesday going through his emails to respond to Mr. Mueller's subpoena, which sought documents related to almost a dozen campaign officials and other advisers, including Trump lawyer Michael Cohen, political consultant Roger Stone and outgoing White House communications director Hope Hicks.
Trump was not pleased to see his former aide, with whom he has had an up-and-down relationship, go on an interview binge, a person familiar with the president's views said Tuesday.
"I thought it was a teachable moment", he said of his 24 hours in the limelight. Other witnesses have presumably testified before Mueller's grand jury since it started meeting last July, but none have made public an appearance.
Nunberg's appearance is a reversal of his pronouncement earlier this week that he would defy a grand jury subpoena issued by Mueller. "Mr. Mueller should understand I am not going in on Friday".
"I think he may have done something during the election", Nunberg told NBC.
"My plan is", Nunberg said, "I'm going to make a plan". The person was not authorized to discuss private conversations publicly.
But in an interview with the Associated Press this week, he said he had worked for hours to produce the thousands of emails requested by Mueller. Among those charged are President Trump's former campaign chairman, Paul Manafort, former White House national security adviser Michael Flynn and 13 Russian Federation nationals accused in a hidden but powerful social media campaign to meddle in the American election.
A spokesman for the special counsel's office declined to comment. Mueller requested communications Nunberg had with several people in Trump's inner circle, including his "mentor" Roger Stone; Stephen Bannon, the former White House chief strategist, and Carter Page, the ex-foreign policy adviser. "What am I supposed to do?" he said of his situation.
When asked by reporters if he would speak following his testimony, Nunberg said "no", according to Politico.
He also insisted that he was sober during the interviews.
He later backed off his defiance and said he would cooperate after all.