Cory Booker shot back at Mike Bloomberg after the former New York City mayor said the New Jersey senator was "well spoken" -- saying he shouldn't have to take him to school on race.
"It's not up to me to be his teacher on this issue," Booker told ABC News' Rachel Scott in an interview Friday afternoon in Grinnell, Iowa. "It's not enough to say, ‘I'm not a racist.' If racism exists, you need to be an anti-racist. You need to be working every day on these issues.
"Whoever is our nominee should not have to be explained to about why comments like that could be found to be offensive to a very important part of our constituency," Booker added.
The back-and-forth between the 2020 presidential hopefuls comes less than two weeks into Bloomberg's entry into the race.
During an interview, Bloomberg, who is white, was asked about the lack of diversity in the Democratic field and the possibility of an upcoming debate featuring only white candidates. Bloomberg said he and Booker endorsed each other a few times.
"He's very well-spoken," Bloomberg said on "CBS This Morning." "He's got some good ideas."
Bloomberg later told ABC News that he "probably shouldn't have used the word."
"But I can just tell you, he is a friend of mine and he is a Rhodes Scholar, which is much more impressive than my academic background," he added, referring to Booker. "I envy him. And he can certainly speak for himself."
Following the suspension of California Sen. Kamala Harris' campaign, Democrats have been met with a shrinking diversity on the campaign trail. As of Saturday afternoon, no candidates of color have qualified for the sixth democratic debate, set to take place in Los Angeles later this month.
Despite a concerted push to qualify for the Los Angeles debate, Booker still needs four Democratic National Committee approved polls to appear on the stage.
In the ABC News interview, Booker criticized the DNC for instituting "artificial barriers" that make it easier for billionaires to get on the stage.
"It's a problem that we now have an overall campaign for the 2020 presidency, that has more billionaires in it than black people," he said in speech to supporters on Thursday in Des Moines, Iowa.
He also pushed back against DNC chair Tom Perez's argument that no candidate who has been under 4% at this point in the cycle has been able to win the primary.
Booker is not alone in urging for more diversity on the debate stage. Fellow presidential hopeful and former Housing and Urban Development Secretary Julián Castro, who is Latino, has also used the conversation to bring attention to his campaign.
The Dec. 19 debate will currently feature former Vice President Joe Biden, Massachusetts Sen. Elizabeth Warren, Vermont Sen. Bernie Sanders, South Bend, Indiana, Mayor Pete Buttigieg, Minnesota Sen. Amy Klobuchar and businessman Tom Steyer.
Hawaii Rep. Tulsi Gabbard and businessman Andrew Yang both need one poll to make the debate stage. Candidates have until Thursday to qualify.
Following his controversial comments, Booker confirmed to ABC News that Bloomberg has reached out to him.
He said he still considers Bloomberg a "friend" and there is "no beef," but Booker spoke of the importance of having leaders who "can authentically connect" to African Americans -- a critical base for the Democratic party.
"We all say things we shouldn't, but this is something our eventual nominee should know," he said. "If they don't it is going to hurt them in our ability to win."
ABC News' Alisa Wiersema, Briana Stewart and Ely Brown contributed to this report.