WATCH: CBS News Democratic Debate

CHARLESTON, S.C. (AP / CBS News) - Democratic rivals are hoping to knock him off his front-runner perch in a debate before a critical South Carolina primary that could dramatically reshape the race.

From left, Democratic presidential candidates, former New York City Mayor Mike Bloomberg, former South Bend Mayor Pete Buttigieg, Sen. Elizabeth Warren, D-Mass., Sen. Bernie Sanders, I-Vt., former Vice President Joe Biden, Sen. Amy Klobuchar, D-Minn., and businessman Tom Steyer, participate in a Democratic presidential primary debate at the Gaillard Center, Tuesday, Feb. 25, 2020, in Charleston, S.C., co-hosted by CBS News and the Congressional Black Caucus Institute. | Source: AP Photo / Patrick Semansky

The Vermont senator is facing an unprecedented assault from his Democratic rivals as the party's turbulent nomination fight threatens to explode on the debate stage Tuesday night.

No fewer than three Sanders rivals unleashed paid attack ads against him. And several outside groups traditionally aligned with Democrats worked to undermine Sanders' standing with key constituencies.

The infighting came just hours before seven candidates were set to meet for the party's 10th and perhaps most consequential debate of the 2020 primary season.

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