Barack Obama called Donald Trump a "fascist" in a phone conversation with Sen. Tim Kaine of Virginia during the 2016 presidential election, Kaine says in a video clip featured in an upcoming documentary about Hillary Clinton.

Kaine, Clinton's running mate on the Democratic ticket, recounts the call during an exchange with Clinton that was caught on camera in 2016. Kaine's wife, Anne Holton, was also present.

"President Obama called me last night and said: 'Tim, remember, this is no time to be a purist. You've got to keep a fascist out of the White House,'" Kaine says before adding with a laugh that Obama "knows me and he knows that I could tend to err."

Clinton replies, nodding, "I echo that sentiment."

She then puts her hands to her chest and says, "But that's really — the weight of our responsibility is so huge."

The clip appears in an episode of "Hillary," a four-part documentary series that will be available on Hulu on March 6. The docuseries, which premieres at the Sundance Film Festival on Saturday at a screening Clinton was scheduled to attend, chronicles her early life, rise to national prominence and political career.

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NBC News was given access to the series this week. Nanette Burstein, an Oscar-nominated filmmaker who directed "Hillary," confirmed in an email that the clip of Kaine was recorded by a camera team hired by the Clinton campaign.

The date and location of the conversation are not clear.

Obama has rarely publicly attacked Trump since leaving office, and his description of Trump as a fascist — as recalled by Kaine — is a far sharper attack than he offered in public during or after the campaign.

In a speech at the 2016 Democratic National Convention, Obama said "homegrown demagogues" were among the political forces threatening American values — a comment some interpreted as a veiled swipe at Trump, then the Republican presidential nominee.

And Obama has criticized some of Trump's policy decisions, such as pulling out of the Paris climate accords, and implicitly jabbed at the president's habits, saying leaders should avoid social media.

Hillary Clinton and 2016 Democratic vice presidential candidate Tim Kaine attend together a campaign rally in Miami on July 23, 2016. Alexander Tamargo / WireImage

Obama's communications director declined to comment about the documentary when contacted by NBC News on Friday. Kaine's office and Clinton's spokesman did not respond to requests for comment.

"Hillary" made headlines this week after The Hollywood Reporter said the series featured clips of Clinton bashing Sen. Bernie Sanders, I-Vt., her opponent in the bitter Democratic primary battle.

"Nobody likes him. Nobody wants to work with him. He got nothing done. He was a career politician," Clinton says in a wide-ranging interview recorded for the series. "It was all just baloney, and I feel so bad that people got sucked into it."

The series features other notable comments from Clinton, who speaks candidly about Trump, as well as her tenure as first lady in the 1990s, President Bill Clinton's affair with Monica Lewinsky, and the various challenges of her years in the national spotlight.

Trump's campaign: "He entered the campaign with such negative energy, so much vitriol … He clearly felt comfortable attacking women, relished in denigrating women."

The second 2016 debate: "(Trump) was stalking me. He was leering over me. He was sort of preening like an alpha male. I knew he was doing it. I was well aware of it, so I was trying to figure out: What do I do? If I wheeled around and I said, 'Back up, you creep, you're not going to intimidate me,' would I sound angry, and would people recoil from that because all he's doing is just standing there?

Her defeat: "I went in and laid down on the bed in the bedroom and I was just, like, bewildered and astonished. I was thinking, 'Wait a minute, what is happening?' I mean, I've been in lots of elections and I didn't see this coming."

The morning after: "I was totally emotionally wrecked. I felt like I'd let everybody down. I felt like … I worried that he wouldn't rise to the occasion, that all the forces he'd unleashed had been rewarded. It made me sick to my stomach. It didn't make sense."

The Lewinsky scandal, Bill's confession: "I was just devastated. I could not believe it. I was so personally hurt and, you know, 'I can't believe this, I can't believe you lied.' It was horrible and I said, 'If this is gonna be public, you've got to go tell Chelsea.'"

Her husband's impeachment: "I had the extra burden of having been on the impeachment staff back in 1974 (during Watergate). I had done the research about what is a high crime or misdemeanor. I knew what the standards of impeachment were. I knew this did not meet it. He shouldn't have done what he did. He shouldn't have tried to hide it. But it was not an impeachable offense."