Juan Williams said Thursday that 2020 presidential candidates must start to differentiate former Vice President Joe Biden from former President Barack Obama in their primary debates if they hope to win over the black vote, and beat President Trump.

"I think a lot of this is about black voters. And you have to understand that part of Biden's appeal and, in fact, the consistent lead that he's held in the polls in this Democratic race, is based on... that he was a very good vice president to the first black president," Williams said on "The Daily Briefing."

"Secondly, he is viewed as the most electable with regard to beating Donald Trump."

"So if you're talking to black voters and you're Kamala Harris, you're Cory Booker, even if you're Bill De Blasio ... you're saying, wait a second, I want to separate him a little bit from President Obama. And to do that, you have to call him on differences."

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Williams said if other Democrats attack Biden for being too tame or bland, they may be able to stir up the party's base and reorient support to their own campaigns.

"You say... we don't like what President Obama did. We think President Obama was far too moderate, far too accommodationist with Republicans, especially now in the era of Donald Trump that Democrats view as extreme. We want more of an angry strong voice, and too often we feel Obama was sort of middling and milk toast," he said.

"Especially if you're talking about Black Lives Matter and some of the younger black activists out there. I'm not sure, by the way, that this works with the older black crowd that is the base for Joe Biden."

Several Obama administration allies, including former White House chief of staff Rahm Emanuel and former attorney general Eric Holder, came out publicly to decry the attacks against Biden's record, comparing it to an attack on the Obama legacy.

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Emanuel told Fox Business Network's "WSJ At Large," in a Friday interview that Obama “is at 90-odd percent among Democrats. He’s the most successful progressive president… since the Great Society.”

Holder tweeted his thoughts on the race on Wednesday and said Democrats were doing themselves a disservice, by not promoting the Obama administration's accomplishments.

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“To my fellow Democrats. Be wary of attacking the Obama record. Build on it. Expand it. But there is little to be gained - for you or the party - by attacking a very successful and still popular Democratic President,” the tweet read.

Fox News' Paul Steinhauser contributed to this report