John Podesta, whose emails were released by WikiLeaks just before the 2016 election, warned before Robert Mueller's congressional testimony that President Trump has "encouraged the destruction of the nation’s fundamental democratic institutions."

The former chairman of Hillary Clinton’s 2016 presidential campaign -- whose emails had been hacked by Russia, according to findings from the U.S. intelligence community -- wrote in the Washington Post that the hearings will show the "raw partisan divide cleaving America" and being worsened by Trump each day.

"Committee members on both sides of the aisle will have to contend with an inescapable conclusion from a fair reading of the report: The Russian attack would not have worked — and perhaps would have been over before it really got started — if not for Donald Trump’s enthusiastic encouragement and amplification of it," he argued.

TRUMP SAYS MUELLER SHOULDN'T TESTIFY, WARNS HEARING WILL BACKFIRE

BRET BAIER: MUELLER HEARING COULD BACKFIRE ON DEMOCRATS

Though Mueller did not allege a conspiracy between the Trump campaign and Russia, Podesta said "it’s clear that every step of the way, Russia was looking for green lights, and at every step of the way, the Trump campaign provided them."

He predicted Russia's "attack on American democracy" will continue in the 2020 election, calling out a "sobering" report by the Pentagon that criticizes “the unwillingness of Western experts and governments to confront the ideological — as well as political and military — aspects of our rivalry with Putinism."

"That’s why Mueller’s testimony is so vital. He can provide guidance on how Russia operates and how to prevent further attacks. But Americans must face the truth: Trump, in broad daylight, has encouraged the destruction of the nation’s fundamental democratic institutions, and he continues to do so," Podesta concluded.

CLICK TO GET THE FOX NEWS APP

A Justice Department official told the former special counsel that his upcoming testimony "must remain within the boundaries" of the public, redacted version of his report, according to a letter obtained by Fox News on Monday.

The letter states: “Dear Mr. Mueller: I write in response to your July 10, 2019 letter concerning the testimonial subpoenas you received from the House Judiciary Committee (HJC) and House Permanent Select Committee on Intelligence (HPSCI). Your letter requests that the Department provide you with guidance concerning privilege or other legal bars applicable to potential testimony in connection with those subpoenas."