Joe Biden warned Wednesday that he’s prepared to go after his Democratic rivals during the third debate scheduled next week, specifically saying he would not be “as polite this time” when it comes to Sen. Kamala Harris.

Speaking to thousands of African-American voters at the NAACP convention in Los Angeles, Biden defended himself against recent attacks from Harris and another 2020 rival, Sen. Cory Booker, regarding his civil rights record, the Los Angeles Times reported.

“I’m not going to be as polite this time,” Biden said, referring to Harris, the California Democrat who during a pivotal moment in the second Democratic debate in June challenged the former vice president over his opposition decades ago to federally mandated school busing.

BIDEN PUSHES BACK AFTER BOOKER JABS AT HIM OVER MASS INCARCERATIONS FOR SECOND STRAIGHT DAY

That same night in Miami, Harris also went on the offensive against Biden, criticizing some comments he had made about his ability to find common ground during the 1970s with segregationist senators with whom he disagreed.

On Wednesday, Biden referred to former President Barack Obama’s decision to pick him as a running mate. Obama “did a significant background check on me for months with 10 people,” Biden said in Los Angeles. “I doubt he would have picked me if these accusations about my being wrong on civil rights is correct.”

Though Biden has been the front-runner among Democratic candidates since he first entered the race, his response to Harris in June and new scrutiny of some controversial episodes from his long career in politics have caused him to lose momentum in polls, the newspaper reported.

During Wednesday’s convention, Booker, a Democrat from New Jersey, also challenged Biden’s support of a 1994 Clinton-era crime bill that critics claim was responsible for the mass incarceration of black men in the U.S.

Biden countered by criticizing the so-called "stop-and-frisk" tactic used against African-American men in Newark, N.J., while Booker was mayor there from 2006 to 2013.

CLICK HERE TO GET THE FOX NEWS APP

“If they want to argue about the past, I can do that,” Biden said. “I got a past I’m proud of. They got a past that’s not quite so good.”

“If they want to argue about the past, I can do that. I got a past I’m proud of. They got a past that’s not quite so good.” — Joe Biden

Biden also highlighted his new, wide-ranging plan to reduce the country’s prison population, reform the nation’s criminal justice system and eliminate racial and income disparities in sentencing.

Biden, Harris and Booker were among 10 presidential candidates to attend the NAACP convention. President Trump’s Republican challenger, former Massachusetts Gov. William Weld, also addressed the convention. Trump declined his invitation, citing scheduling issues, the newspaper reported.

Twenty Democratic candidates will face off over two nights at the Fox Theater in Detroit in the next round of the party's debates next Tuesday and Wednesday. Biden, Harris and Booker will each participate in the Wednesday debate.

Fox News’ Paul Steinhauser contributed to this report.