"Specifically now, and in light of learning, one that Michael was taping the president, two, that he doesn't have a tape of this, I would believe the president", he said.
Giuliani's recent attacks against Cohen's character have been in stark contrast to his prior praise for Cohen.
"The man is a pathological manipulator, liar", Giuliani said.
Nunberg said on NBC's "Meet the Press" that it was common knowledge among Trump's associates that Cohen recorded phone calls.
The Fox host spoke with Sandra Smith on America's Newsroom Friday and shared his thoughts on Cohen's claim that the president approved Donald Trump Jr. Michael Cohen alleges that then-candidate Donald Trump knew in advance about a controversial June 2016 Trump Tower meeting where Russians offered to give damaging information on Hillary Clinton.
He also confirmed that the joint defense agreement between Trump and Cohen has ended. The Justice Department has been investigating Cohen for months, raiding his home, office and hotel room in search of documents related to a $130,000 payment the attorney facilitated before the 2016 election to Stormy Daniels, an adult-film actress who says she had sex with Trump in 2006.
He similarly told ABC News in early July, "I have no concerns that Michael Cohen is going to do anything but tell the truth".
For more than a year, both the president and his son have adamantly denied that Trump was aware of the meeting at the time.
"He expressly waived attorney client privilege last week and repeatedly and inaccurately - as proven by the tape - talked and talked about the recording, forfeiting all confidentiality", Davis said.
Mentzer also cited a letter sent in February by Senate Democrats to Chuck Grassley, the committee's Republican chairman, saying that "Feinstein and other committee members have called multiple times for Donald Trump Jr.to appear before the committee".
CNN reported Thursday Trump's former longtime lawyer and fixer, Michael Cohen, claims Trump knew in advance about the meeting. "But vis-a-vis his professional relationship with the president, this is, I mean it's highly unethical, if not, if not, you know, being - getting disbarred for what he did".
He said he does think Trump could have known about the meeting after the fact.
Jay Goldberg, another former Trump lawyer, argued Friday on CNN's "Erin Burnett Out Front" that Giuliani "immeasurably" damaged his case against Cohen with the sudden shift in tone. Earlier this month, however, he said that his loyalty is first to his family and country, not the President.