WASHINGTON (Reuters) - At various stages of their presidential campaigns, Joe Biden and Pete Buttigieg have each faced a central question: Can a Democratic Party that increasingly prizes diversity and progressivism support a moderate white man as the nominee?

FILE PHOTO: Democratic 2020 U.S. presidential candidate and former Vice President Joe Biden speaks at an event at the Mississippi Valley Fairgrounds in Davenport, Iowa, U.S. June 11, 2019. REUTERS/Jordan Gale/File Photo

Both candidates revived that debate this week, when they ran up against the issue of race, spotlighting their relationship with the African-American community, a vital Democratic constituency.

For Biden, it was his boast of working with avowed segregationists in the Senate in the name of civility and getting things done. For Buttigieg, it was dealing with the aftermath of the fatal police shooting of a black man in South Bend, Indiana, where he serves as mayor.

The controversies again raised questions about whether either is the best choice to galvanize black voters as the Democratic nominee against Republican President Donald Trump in the November 2020 presidential election.

Democrats have emphasized the importance of African-American voters for next year’s election after black turnout dipped in 2016 when Hillary Clinton was the Democratic nominee.

“There is a huge elephant in the room this cycle - which is race,” said Quentin James, executive director of The Collective PAC, which is working to ensure a black candidate is on the Democratic ticket.

At a fundraiser earlier this week, Biden, 76, talked of working as a young senator with Southern Democrats Herman Talmadge and James Eastland.

While discussing Eastland, a senator from Mississippi who described black people as inferior and fought against efforts to desegregate the South, Biden said: “He never called me boy, he always called me son.”

He was swiftly criticized by several other presidential candidates, including U.S. Senators Cory Booker, Kamala Harris and Elizabeth Warren.

When Booker, an African-American who is almost 30 years younger than Biden, called on him to apologize, Biden responded by defending his civil rights record and saying Booker should apologize to him. “There’s not a racist bone in my body,” he said.

Booker was visibly upset in an interview with CNN afterward, saying Biden, who served two terms as vice president to Barack Obama - America’s first black president - had missed the point about the harm his words caused.

“What matters to me is that a guy running to be head of our party, which is a significantly diverse and wondrous party, doesn’t understand or can’t even acknowledge that he made a mistake,” Booker said. “He knows better.”

The front-runner for the Democratic nomination, Biden has long championed civil rights and was honored earlier this year by Al Sharpton’s National Action Network. At a fundraiser on Wednesday, he cited civil rights leader Martin Luther King Jr. as an inspiration for entering public service and spoke of fighting segregation alongside the late Senator Edward Kennedy.

But James argued the base of the Democratic Party had shifted beneath Biden’s feet. Since his campaign began, Biden has strived to persuade progressive elements of the party that he is in step with them on issues such as abortion and climate change. This week’s flap “continues a narrative for Biden that he is not in line with the future of the party,” James said.

Nancy Pelosi, who as speaker of the House of Representatives is the top elected U.S. Democrat, defended Biden on Thursday, calling him “authentic.”

“Joe Biden seems to have tremendous support in the African-American community. But it’s for them to decide, it’s not for me to make a judgment as to how they’re going to react to him,” she said.

RAW RELATIONS

Buttigieg has spent the week dealing with a crisis in South Bend, after a white police officer shot a black man early on Sunday who he said had flashed a knife. The officer’s body camera had been switched off. Buttigieg canceled campaign events and returned home.

He has had at times a rocky relationship with the city’s black residents, stemming from his decision to fire the African-American police chief early in his mayoral tenure and criticism that poorer neighborhoods have not benefited from his efforts to revitalize the local economy.

Openly gay, Buttigieg, 37, has been an effective fundraiser in part because of backing from the LGBTQ community. But he has acknowledged he has largely failed to win over black voters in critical early voting states such as South Carolina.

“We know it’s going to take extra work because I’m not from a community of color and also was not a famous person when this process began,” Buttigieg said last week in Charleston, South Carolina. “We’re working very energetically, very actively, in order to invite more people and specifically black voters into this campaign.”

Bolstered by his partnership with Obama, Biden has long enjoyed strong support from black voters in South Carolina and elsewhere.

Antjuan Seawright, a Democratic strategist in Columbia, South Carolina, said that while that was unlikely to change, Booker and Harris, another African-American candidate, could now siphon off some of that support.

He said Biden erred in attacking Booker, who trails him significantly in opinion polls. “You don’t punch down,” he said.

Buttigieg, Seawright said, was in deeper trouble. “He’s polling at zero percent,” with black voters in the state, he said. The trouble in South Bend, “is just another lump in his political carpet.”