'The American people are due the truth': Ex-Nixon adviser Gergen says Trump must testify to Mueller

David Gergen, a former adviser to the late President Richard Nixon, told CNN's John King on Wednesday that President Donald Trump can no longer refuse to testify under oath to special counsel Robert Mueller.

While breaking down the news that former campaign chairman Paul Manafort and former Trump attorney Michael Cohen will be going to prison, Gergen said that silence is simply not an option anymore.

"He has to go before Mueller now and testify," Gergen said. "The American people are due the truth on this. We have, as Rudy Giuliani keeps pointing out, competing truths, competing narratives about what happened. We haven't heard the president's side of this story under oath."

Gergen said it was particularly important for Trump to testify under oath given that he's already been caught lying about knowing anything about Cohen's scheme to make illegal campaign contributions as a way to pay off women who claimed to have had affairs with Trump.

The former Nixon adviser then said that Trump should publicly rule out ever pardoning Manafort or anyone else caught up in the Russia probe.

"If he sent a clear message on that, it would be very reassuring," Gergen said. "As it is now, there's no basis on which to believe anything else the president has said, absent of full investigation for Mueller."

Watch the video below.