Former US Vice-President Joe Biden has come under fire after recalling his working relationship in the 1970s with segregationist senators as "civil".
"He never called me 'boy,' he always called me 'son,'" he said referring to one of the southern Democrats at a fundraising event on Tuesday night.
His comments were quickly panned by rival candidates and critics who noted the racial undertones of the comment.
Early polls have shown Mr Biden leading the crowded 2020 presidential field.
What did Biden say?
Speaking at the Carlyle Hotel in New York City on Tuesday night, Mr Biden praised the "civility" amongst lawmakers in the 1970s when he first joined the US Senate.
He called out his work with two southern Democrats who used their political position to advance their beliefs that white and black people should not live together.
"I was in a caucus with James O Eastland," Mr Biden told the audience, referring to the segregationist senator from Mississippi.
He added that Eastland never referred to him as "boy" during their working relationship.
The Washington Post, and other outlets, have pointed out that "boy" is a racial epithet used to describe black men. Mr Biden's campaign have yet to comment on why it would be unusual for the former senator to not call Mr Biden - who is white - "boy".
Mr Eastland notably referred to African Americans as an "inferior race" and warned that racial integration would lead to "mongrelisation".
Referring to late Georgia Senator Herman E Talmadge - who opposed the desegregation of US public schools, Mr Biden said: "We got things done. We didn't agree on much of anything. We got things done."
Joe Biden has been preaching civility and bipartisanship as a means for transcending petty politics, but for some Democrats this may be getting a bit ridiculous. It's one thing to talk about working across the political aisle. It's another, however, to boast about co-operating and socialising with unreconstructed southern segregationists.
Yet that's exactly what Mr Biden did in a fund-raiser on Tuesday, in an effort to illustrate his view that Washington operates best when even bitter political disagreements don't lead to lasting enmity.
It's a point that may be worth making, and may appeal to a broad spectrum of more moderate Democratic voters, but the way he did it - citing specific long-deceased politicians with troubling records on racial justice - is dynamite for the party's activist base.
Even if Mr Biden eventually comes out on top, casual remarks like these - combined with past views on abortion, crime policy and financial regulation - could make him a difficult candidate for some Democrats to rally behind.
The former vice-president has a history of gaffes that have derailed his past presidential bids. There was some suggestion that Mr Biden may be more careful this time around. Early indications are that will not be the case.
The Joe Biden America has seen is the Joe Biden America will get.
How have Democrats reacted?
Senator Cory Booker responded: "You don't joke about calling black men 'boys.'"
"Vice President Biden's relationships with proud segregationists are not the model for how we make America a safer and more inclusive place for black people, and for everyone," the presidential candidate from New Jersey said in a statement.
"I'm disappointed that he hasn't issued an immediate apology for the pain his words are dredging up for many Americans. He should."
Fellow candidate and New York City Mayor Bill de Blasio, took to Twitter to share a photo of his inter-racial family.
It’s 2019 & @JoeBiden is longing for the good old days of “civility” typified by James Eastland. Eastland thought my multiracial family should be illegal & that whites were entitled to “the pursuit of dead n*ggers." (1/2) pic.twitter.com/yoOOkpaTX2— Bill de Blasio (@BilldeBlasio) June 19, 2019
"It's past time for apologies or evolution from Joe Biden," Mr de Blasio added.
"He repeatedly demonstrates that he is out of step with the values of the modern Democratic Party."
Another 2020 rival, former Maryland congressman John Delaney also weighed in.
"Evoking an avowed segregationist is not the best way to make the point that we need to work together and is insensitive; we need to learn from history but we also need to be aggressive in dismantling structural racism that exists today," Mr Delaney tweeted.
Mr Biden's campaign has suffered numerous political gaffes including plagiarism, accusations of inappropriate touching and praise for Christian evangelical Vice-president Mike Pence.
After initially praising the Indiana Republican as a "decent guy", he immediately reversed after a backlash from the LGBT community.
"There is nothing decent about being anti-LGBTQ rights, and that includes the vice president," he told a Twitter user who had criticised him in comments.