Sen. Bernie Sanders, I-Vt., said Wednesday that former Vice President Joe Biden, the presumptive Democratic presidential nominee, must work on strengthening his appeal to working-class and young voters ahead of the November election.

"I think there is an excellent chance that Biden is going to win," Sanders told "MSNBC Live," "but in order to guarantee that, he is going to have to reach out to working class people and young people in a way he has not done up to now.

SANDERS ADVISER WARNS 'A SIGNIFICANT PORTION' OF SENATOR'S BACKERS 'UNSUPPORTIVE' OF BIDEN

"And he is going to have to make it clear: He stands with a working class in this country that is struggling right now. He is prepared to take on the corporate elite and the big money interests in order to create an economy that works for all, not just a few."

Sanders suspended his presidential campaign last month after Biden rattled off a succession of primary victories. Although the self-described democratic socialist has endorsed Biden, the former vice president represents a stark contrast to the kind of progressive politics Sanders and others favor. That became particularly apparent during the primary race when Biden sparred with his Democratic rivals over "Medicare-for-all," an expansive government takeover of health care that Sanders has long supported.

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Securing the votes of the left flank of the Democratic base has been a top priority for the Biden campaign over the past month.

“We've listened to Bernie supporters,” the former vice president said said in an interview last week with Nevada TV station KLAS.

Meanwhile, a top Sanders adviser has warned that a good portion of the Vermont senator's supporters aren't yet willing to back the former vice president.

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Jeff Weaver – who managed Sanders’ 2016 campaign and served as senior adviser on his now-suspended 2020 White House bid – cautioned, “The Bernie Sanders base makes up a sizeable group of voters that could have a profound impact in the outcome of the election.”

Weaver raised the red flags in a four-page document issued by his newly formed "America’s Promise" super PAC that was first reported by NBC News and confirmed by Fox News.

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“There is a real and urgent need to help Biden consolidate Sanders supporters,” Weaver said. “If all of Sanders’ base turned out for Joe Biden in November, he could defeat Trump and take back the White House for Democrats. Here’s the problem: significant portions don’t currently plan to.”

He said these supporters of Sanders are “currently unsupportive and unenthusiastic” about Biden.