President Trump's attempts to discredit the Russia investigation and his attacks against the media pose a "growing" threat to American democracy, so says David Gergen.

Gergen is a CNN political analyst who served as a presidential adviser in four administrations, including Richard Nixon's, Gerald Ford's, Ronald Reagan's, and Bill Clinton's. He said on CNN's "Reliable Sources" Sunday that Trump's comments may succeed in keeping his base together, but it's also "eroding" America's "sense of unity."

His remarks come after a steep escalation in Robert Mueller's Russia investigation last week. The special counsel indicted 13 Russian nationals and three Russian companies that are accused of attempting to manipulate the 2016 election in favor of Trump. The president has responded by asserting the indictment shows his campaign did not collude with Russia. Trump also continued his attacks on the press.

White House deputy press secretary Hogan Gidley doubled down on Trump's claims on Fox News Saturday.

"This makes it clear and concise for the American people and proves the president correct: No collusion between Donald Trump, his campaign and Russia," Gidley said. "There are two groups that have created chaos more than the Russians, and that's the Democrats and the mainstream media."

Related: Mueller indicts 13 Russian nationals over 2016 election interference

Gergen said this type of rhetoric can have profound consequences.

"We have an election that's just around the corner, later this year," Gergen told host Brian Stelter. "What if the Democrats, you know, take the House back? Is that all going to be blamed on 'Fake News?' What if they fall short? Are Democrats going to think, 'You know, it was all that meddling.'"

"This is what splits a country apart," he added. "In many other countries, it's been the beginnings of an authoritarian rule. And that's the larger threat hanging over us now...I find that the threat is growing."

Gergen added that Trump's recent behavior -- including a barrage of erratic tweets posted Sunday morning -- is reminiscent of former-President Nixon's just before he resigned in 1974 amid the Watergate scandal.

"It reminds me very much of the last days of President Nixon when he became deeply, deeply insecure, lashed out in all sorts of ways and didn't remain focused on the job at hand," Gergen said. "This, I'm afraid, has been President Trump's pattern for some time."

Gergen added that, to him, Trump's tweets suggest that he is concerned the ongoing investigation into Russian meddling will dredge up information "he very much does not want us to know."

Dan Pfeiffer, a former senior adviser to President Obama and a CNN political commentator who also appeared on "Reliable," said he too believes "our democracy is under assault -- and that assault is coming from 1600 Pennsylvania Avenue."

"We think our democracy can survive anything, but these things happen slowly and then suddenly," Pfeiffer said.