Speaking first to the Washington Post and then on MSNBC, Nunberg vowed to defy Mueller, who is investigating Russian meddling in the 2016 US election and alleged collusion between Trump aides and Moscow.
Mueller has indicted 13 Russians and four former Trump aides, three of whom have entered plea deals involving co-operation.
Nunberg, however, said he would tear up his subpoena live on Bloomberg TV.
He also told MSNBC host Katy Tur, the author of a bestselling book on the Trump campaign, that he thought the candidate “may have done something” illegal during the election.
He added: “I don’t know that for sure.”
Nunberg, a protégé of veteran political operative Roger Stone, was Trump’s political adviser prior to the start of his White House run.
He was fired in August 2015, over racially charged Facebook posts, after he and Stone lost a internal power struggle with then-campaign manager Corey Lewandowski.
Nunberg was sued by Trump on the eve of the 2016 Republican convention, for allegedly leaking information about Lewandowski’s relationship with close Trump aide and future White House communications director Hope Hicks. However, Nunberg has remained close to many in Trump’s orbit.
Speaking to the Post, Nunberg dared Mueller to act if he refused to appear before a grand jury on Friday.
“Let him arrest me,” he said.
Speaking to MSNBC, Nunberg said: “I think it would be funny if they arrested me.”
Nunberg told Tur he would not co-operate with Mueller, saying: “It’s a witch hunt and I’m not going to cooperate.
“Why do I have to spend 80 hours going over my email? That I’ve had with Steve Bannon and Roger Stone? Why does Bob Mueller need to see my emails when I send Roger and Steve clips and we talk about how much we hate people?”
Nunberg also said that had Trump not won the Republican primary, “he was probably going to endorse Hillary Clinton”.
He also showed the Post a copy of what appeared to be his subpoena, the newspaper reported, which included a list of names of those about whom the special counsel is seeking information. Hicks, former White House strategist Steve Bannon, Trump lawyer Michael Cohen, Lewandowski and Stone were among the names listed.
White House press secretary Sarah Sanders dismissed Nunberg’s comments.
“I’m not going to weigh into someone who doesn’t work at the White House,” she said. “From our perspective, we’re going to cooperate with the special counsel’s office and the reason we’re so comfortable doing so is there was absolutely no collusion with the Trump campaign.”
Nunberg also asked Tur for advice, saying: “What do you think Mueller is going to do to me?”
Tur responded: “I’m not a lawyer, I don’t know but given the circumstances you might be held in contempt of court.”
In a separate television interview, Nunberg told CNN that Mueller “thinks Trump is the Manchurian Candidate” though the former aide said he disagrees.
Nunberg said he believes investigators are interested in learning more about the Miss Universe pageant Trump held in Moscow in 2013.
“They probably want to know about Miss Universe 2013, if I had to guess,” Nunberg said. “There was nothing there, but they want to hear the testimony. They want to hear what other people said, and perhaps other people told them different things than I heard.”
It was during this visit to Moscow that Trump is said to have discussed potential business opportunities in Russian. Nunberg said he was asked about Trump Tower Moscow, which was under discussion but never built.
Investigators also asked him about the goings on inside Trump Tower in New York, where Trump lived and worked before moving into the White House.
“They asked questions to me in terms of did I hear Russian spoken around Trump Tower? No, Gloria, I never heard Russian spoken around Trump Tower, OK? Now, I understand why they have to ask that, but it was pretty ridiculous to me,” Nunberg said, referring to CNN’s political analyst, Gloria Borger.
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