The MSNBC interview came after The Washington Post published a story that cited Nunberg as saying he would refuse to comply with the subpoena. "Let him arrest me," Nunberg told the newspaper. "Mr. Mueller should understand I am not going in on Friday."
Nunberg said that he was refusing the special counsel "because what they sent me was absolutely ridiculous."
"Should I spend 50 hours going over all my emails with [Trump confidant and longtime political operative Roger Stone] and with Steve Bannon?" Nunberg said in the MSNBC interview, adding that Mueller's team also asked for communications with former Trump campaign staffers Hope Hicks and Corey Lewandowski. Last week, the White House said Hicks, a longtime close aide to Trump, would be leaving her role as communications director.
Despite being "not a fan of Donald Trump" for the way Nunberg says he treated Stone and others, the former Trump aide said that "when they get a subpoena like this ... Mr. Trump's right, it's a witch hunt."
But when asked whether he believed that the special counsel may have something on Trump, Nunberg responded, "I think they may."
He added: "I think that he may have done something during the election. But I don't know that for sure."
Nunberg softened his stance somewhat in a later interview with CNN.
"I suspect that they suspect something about him," he told the network in an interview. "I can't explain it unless you were in there. ... The way they asked about his business dealings, the way they asked if you heard anything even while I was fired ... it just made me suspect that they suspect something about him."
CNN also asked whether investigators sought information about plans for a Trump Tower Moscow. "Yes," Nunberg said.
In a later CNN interview on Monday, Nunberg alleged that Trump knew about the now-infamous June 2016 meeting between Trump campaign officials — including Donald Trump Jr., Paul Manafort and Jared Kushner — and reportedly Kremlin-connected Russians, who promised damaging opposition research on then-candidate Hillary Clinton.
"He talked about it a week before," Nunberg said of Trump. "All he had to say was, 'Yeah, we met with the Russians. The Russians offered us something, and we thought they had something and that was it.' I don't know why he went around trying to hide."
White House staff initially denied the meeting involved the president in any way, but a Washington Post report later alleged that Trump was directly involved in crafting the initial, misleading statement claiming that the meeting was centered around Russian adoption.
The Associated Press reported in December 2017 that Trump Jr. testified before the Senate Intelligence Committee that he had not told his father about the meeting at the time it happened.