"They have reached a critical mass in their investigation of the secretary and all of her senior staff," DiGenova said. "And, it's going to come to a head, I would suggest, in the next 60 days."
FBI Director James Comey has remained tight-lipped about the investigation into whether Clinton and her staff mishandled classified information on a private email server during her time as the US’ top diplomat.
The Clinton campaign insists the investigation is not criminal in nature, and the FBI has yet to interview the presidential candidate about the issue.
DiGenova explained that the decision on whether or not to indict will fall on Attorney General Loretta Lynch, and will be very complicated politically.
"I believe that the evidence that the FBI is compiling will be so compelling that, unless [Lynch] agrees to the charges, there will be a massive revolt inside the FBI, which she will not be able to survive as an attorney general. It will be like Watergate. It will be unbelievable," DiGenova said.