Joe Biden’s comments about his past work with segregationists “evoked a terrible power dynamic” and showed “a terrible lack of understanding”, rival presidential contender Cory Booker said on Sunday.

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At a New York fundraising event on Monday, Biden spoke about his work as a senator from Delaware with two racist Democrats, James Eastland of Mississippi and Herman E Talmadge of Georgia.

He disagreed with them, he said, but worked with them and “got things done … with some civility”. Reportedly assuming an imitation southern drawl, he also said Eastland “never called me ‘boy’, he always called me ‘son’”.

In a southern context, “boy” is a racist and demeaning form of address used by white people to black men.

Booker, who is African American, led criticism of Biden’s remarks and said the former vice-president should apologise. Biden retorted: “Cory should apologise. He knows better. There’s not a racist bone in my body. I’ve been involved in civil rights my whole career. Period.”

The former vice-president and the New Jersey senator spoke on Wednesday evening and both attended events hosted by African American Democrats in South Carolina this weekend, during which Biden told MSNBC host and civil rights leader Rev Al Sharpton his remarks had been taken out of context.

There’s not a racist bone in my body. I’ve been involved in civil rights my whole career. Period Joe Biden

“I do understand the consequence of the word ‘boy’, but it wasn’t said in any of that context at all,” Biden said. He did not apologise but said “to the extent that anybody thought I meant something different, that is not what I intended”.

On Sunday, Booker told ABC’s This Week: “I’ve said my piece. I have a lot of respect for Joe Biden and a gratitude towards him, and … I have to … be candid with him, to speak truth to power.

“He is a presidential nominee and to say something – and again it’s not about working across the aisle, if anything I’ve made that a hallmark of my time in the Senate to get big things done and legislation passed.

“This is about him evoking a terrible power dynamic that he showed a lack of understanding or insensitivity to by invoking this idea that he was called ‘son’ by white segregationists who, yes, they see him, in him, their son.”

Booker said he did not understand Biden’s claim to have been taken out of context, as he had “listened to the full totality of what he was talking about and frankly I heard from many, many African Americans who found the comments hurtful.

“Look, we make mistakes, we sometimes tread upon issues that maybe we aren’t knowledgeable of. I don’t think the vice-president should need this lesson but this was a time for him to be healing and to be helpful especially the time that he is looking to bring this party together and lead us in what is the most important election of our lifetime.

I heard from many, many African Americans who found the comments hurtful Cory Booker

“And I was disappointed, I’ve said my piece, we had a very constructive conversation, again I have a tremendous amount of respect and appreciation for the vice-president. That’s why again I felt it really important especially with our friends not to just sweep things under the rug but to be candid and straightforward with each other.”

On CBS’s Face the Nation on Sunday another contender for the Democratic nomination, the Vermont senator Bernie Sanders, was given his own chance to be candid and straightforward.

“If your question is [do] I think Joe Biden is a racist?” he asked. “Absolutely not. No I don’t. Not for a second. Joe is a friend of mine. I like Joe and I hope very much that this campaign will be about the real issues facing the American people and not, you know, ugly attacks.”

But he also said he thought “Joe owes the country an apology … and that it is one thing to work with people in the Senate as you have to do, as every senator does, I do, with people who have fundamental disagreements with. That’s one thing. You do that. That’s your job.

Facebook Twitter Pinterest Cory Booker passes Joe Biden at Jim Clyburn’s World Famous Fish Fry in Columbia, South Carolina. Photograph: Leah Millis/Reuters

“But it’s another thing to kind of extol those relationships. You cannot be extolling people who really were part of a disgusting system that oppressed and terrorised millions of African Americans.”

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Attacks on Biden over his long record as a legislator – including a stand in the 1970s against busing to end school segregation – gaffes made in two presidential runs and behaviour towards women have been a feature of the run-up to the Democratic debates.

In Miami this week Booker will take the stage in the first debate on Wednesday and Biden will be part of the second, on Thursday.

Biden leads most polls of the 23-strong field from its ideological centre. The realclearpolitics.com national polling average puts him 16.9% clear of Sanders in second. The same average places Booker seventh on 2.3%, behind Elizabeth Warren in third, Pete Buttigieg, Kamala Harris and Beto O’Rourke.

Harris also spoke to CBS on Sunday. Asked about Biden’s remarks, the California senator said she was troubled by his “praising and coddling individuals who made it their life’s work and built their reputation off of segregation of the races”.

Harris, whose mother was born in India and whose father came to the US from Jamaica, added: “I would not be a member of the United States Senate if those men that he praised had their way.

“We cannot be ignorant of the history of race in this country. And certainly anyone who is a leader should not be.”