Former Vice President Joe Biden Joe BidenPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Fox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio MORE greeted his fellow presidential contender Sen. Kamala Harris Kamala HarrisHundreds of lawyers from nation's oldest African American sorority join effort to fight voter suppression Biden picks up endorsement from progressive climate group 350 Action 3 reasons why Biden is misreading the politics of court packing MORE (D-Calif.) at the Democratic debates on Wednesday by telling her to "go easy" on him.

"Go easy on me, kid," Biden said jokingly as he and Harris shook hands on stage.

Harris' national press secretary, Ian Sams, questioned Biden's comments on Twitter, asking why the former vice president referred to the senator as "kid."

The rematch comes over a month after Harris confronted Biden over his civil rights record as a senator, specifically on busing.

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The episode resulted in a boost for Harris in the polls and fundraising, catapulting her into the top tier of the crowded Democratic primary.

Biden, who has been a front runner in the race since he entered in April, was put on the defensive, and said he didn't expect Harris to go after him the way that she did.

"I was prepared for them to come after me, but I wasn't prepared for the person coming at me the way she came at me," Biden told CNN's Chris Cuomo in an interview.

The former vice president has also been on the defensive from Sen. Cory Booker Cory Anthony Booker3 reasons why Biden is misreading the politics of court packing Bipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility MORE (D-N.J.), who has hit Biden on criminal justice reform.

Booker, who has struggled to gain traction in the primary, is expected to aim for a breakout moment during the debate.

Biden will be flanked by Booker and Harris during the second debate.