Joe Biden has revealed that his campaign has collected dirt on his Democratic presidential nominee rivals - but claims he will never use it against them.

Referring to the crowded field of White House hopefuls, the former vice president said: 'I mean, I get all this information about other people's pasts, and what they've done and not done.'

Biden, who has seen his initial poll lead slashed following a lukewarm performance in the televised debates, told CNN on Friday: 'I'm just not going to go there.'

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Biden, who has seen his initial poll lead slashed following a lukewarm performance in the televised debates, told CNN on Friday: 'I'm just not going to go there'

In May, Biden, 76, promised not to attack his rivals, saying: 'I will not speak ill of any of the Democratic candidates, I will not do it.'

But the veteran politician may now have hinted that he isn't the only candidate who deserves a grilling over previously held positions.

His comments came in the wake of Sen. Kamala Harris' attack on Biden during the opening Democratic National Committee debate in Miami.

Kamala Harris makes a point as she campaigns Kamala Harris, US Presidential Election Campaigning, Sioux City, Iowa - 05 Jul 2019

Despite the thinly veiled jibes towards his rivals, it remains unlikely that Biden would pivot so radically from his affable 'Uncle Joe' persona which positioned to be seen as an antidote to Trump's aggressive posturing (two of Biden's rivals from left, O'Rourke and Buttigieg, pictured)

She took him to task for his opposition to federally mandated busing in the 1970s, a policy aimed at encouraging educational integration.

Kamala also blasted controversial remarks Biden made about working with known segregationists in the Senate at the same time.

Despite the thinly veiled jibes towards his rivals, it remains unlikely that Biden would pivot so radically from his affable 'Uncle Joe' persona which positioned to be seen as an antidote to President Trump's aggressive posturing.

Democratic presidential candidate Sen. Elizabeth Warren

Democratic presidential candidate Sen. Bernie Sanders

Before the attacks from Harris and New Jersey Sen. Cory Booker on Biden for his segregationist comments, the Democratic race for the White House had been relatively genteel.

There are now just seven months still until the Iowa caucuses and the former vice president has repeatedly said on the campaign trail that he is seeking the presidency because he wants 'to restore the soul of this country' and 'unite' the nation.

Biden has also pledged to support the party's eventual leader who will be anointed at the Democratic National Convention next July in Milwaukee, Wisconsin.

Meanwhile, Biden admitted Friday he wasn't ready to be attacked on the busing issue by Harris during last week's Democratic debate.

'Sure they were going to come after me. 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 of the debate confrontation. 'She knows me.'

He argued it's easy for someone to go back and take something in his long public record out of context.

'What I didn't see was people who know me. They know me well. It's not like somebody who just came out of the blue and didn't know anything. It's easy to go back 30, 40, 50 years, and take a context completely out of context,' he said.

He said he wouldn't attack rivals on their past.

'I get all this information about other people's past and what they've done and not done. I'm not going to go there,' he said.

The former vice president also tried to clarify his position on the busing issue.

'I was for voluntary bussing,' he said. 'The way to deal with that problem is what I did from the time I was a kid. I got out of law school, came back, had a great job as a public defender. I fought for putting housing in low-income housing in suburbia. I talked about eliminating red lining. I talked about school districts should be consolidated in ways that made sense.'

CNN host Chris Cuomo asked him: 'Why didn't you fight it like this in the debate?'

Biden blamed the 30 second time limit, which is the amount of time the candidates were allowed to respond.

'What didn't want was to get in the scrum. Do you think the American public looked at that debate, take me out of it, and thought, boy, I really liked the way that's being conducted?,' he said.