Former Vice President Joe Biden visited an African-American church in Birmingham, Alabama, and said white people "can never fully understand" institutional racism in the US.

He was visiting the church on the 56th anniversary of a Ku Klux Klan bombing there.

"We must acknowledge that there can be no realization of the American Dream without grappling with the original sin of slavery," Biden told the congregation from the pulpit.

Biden, a frontrunner in the race to secure the 2020 Democratic nomination, appears to poll favorably with black voters.

However, his recent debate performance prompted criticism about his prior statements on the legacy of slavery and his views on African-American communities.

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Former Vice President and 2020 candidate Joe Biden visited a black church in Birmingham, Alabama, on Sunday and told the congregation that white people "can never fully understand" institutional racism in the United States.

He had been visiting the 16th Street Baptist Church to mark the 56th anniversary of a Ku Klux Klan bombing there, which killed four black girls.

In his 20-minute speech, Biden made reference to recent attacks by white supremacists in the US, saying: "The domestic terrorism of white supremacy has been the antagonist of our highest ideals since before the founding of this country. Lynch mobs, arsonists, bomb makers, lone gunman."

"And as we all now realize, this violence does not live in the past," he said.

"Those of us who are white try, but we can never fully, fully, understand no matter how hard we try."

He also acknowledged that the legacy of slavery remains in the US, saying: "We must acknowledge that there can be no realization of the American Dream without grappling with the original sin of slavery."

Former Vice President Joe Biden at the the 16th Street Baptist Church in Birmingham, Alabama, on September 15, 2019. Joe Biden/YouTube

Biden has made the threat of white supremacy a key part of his campaign. In a video announcing his candidacy for the 2020 presidency this April, he explicitly singled out President Donald Trump's reaction to a white nationalist rally that turned violent in Charlottesville, Virginia, in 2017.

Read more: Joe Biden's 2020 announcement seized on images of Charlottesville violence to declare a battle for the soul of the nation

Biden's appearance at the church also highlights his deep connections to the African-American community.

Since serving as vice president under Barack Obama, Biden has consistently polled highly with African-Americans in the country. According to a recent CNN/SSRS poll, 42% of black voters are supportive of Biden, compared to 21% of white voters in the same study.

In South Carolina, a state that offers early voting and is home to a high proportion of African-American voters, Biden polls favorably with 51% of black voters.

Biden and fellow 2020 contender Elizabeth Warren debating. Mike Blake/Reuters

But Biden caught heat on Thursday during the Democratic primary debate in response to a question on how the government should address slavery.

Noting that Biden in 1975 said "I'll be damned if I feel responsible to pay for what happened 300 years ago," ABC News Correspondent Linsey Davis asked: "You said that some 40 years ago, but as you stand here tonight, what responsibility do you think that Americans need to take to repair the legacy of slavery in our country?"

Biden began his response by admitting that there is "institutional segregation in this country," but trailed off into an answer about better education for poor communities, and suggested that black parents need outside intervention to raise their children.

He also said that playing the radio or "record player" would help poor children acquire language, in reference to a 2017 study which found that children from low-income households hear fewer words than wealthy kids.

TIME editor Anand Giridharadas said that Biden's answer perpetuated racist stereotypes against African-Americans, and was "appalling and disqualifying" for his campaign.

Read more: Joe Biden responded to a debate question about slavery and reparations by going off on a tangent about how black parents should raise their kids, and people aren't happy about it

Biden's communications director Kate Bedingfield said in an interview with MSNBC that but said she did not believe Biden should have answered the answer differently, and highlighted Biden's education plan to increase federal spending on schools and districts in low-income communities.

"I think he very clearly expressed that this is a priority for him, that it's something that's motivated him his entire career," she said.