Expect bare-knuckle political sparring when 2020 frontrunner Joe Biden battles his Democratic rivals this week, with challengers criticizing him on racial issues and low-polling candidates seeking last-gasp breakouts for their sagging campaigns.

Twenty Democratic White House hopefuls will gather under the klieg lights for their party’s second nationally-televised debate, a two-night affair beginning Tuesday in Detroit that is expected to bring more heat than the debut showdown last month.

“I’m not going to be as polite this time,” Biden told a fundraiser in Detroit Wednesday, as the two most prominent African American candidates in the race — senators Kamala Harris and Cory Booker — signalled they will seize the offensive against the former vice president.

Despite differences over how to go after Republican President Donald Trump, and recognition that issues such as the economy and health care should dominate campaign talk, the escalating conflict over race has emerged as perhaps the prominent issue of the moment ahead of the debate.

Trump himself has fueled the divisions, attacking ethnic minority lawmakers in tweets and campaign appearances that sparked fraught national deliberations about race relations in America.

With Biden the clear leader in Democratic polls — he has double the support of number two candidate Bernie Sanders — he is also the primary target.

He will face open confrontation about his record when he takes the stage Wednesday for his rematch with Harris, which will also feature Booker.

Both have pointed to controversial positions in Biden’s past, including his work with segregationists in the US Senate and his opposition to federally mandated busing to desegregate schools.

“If they want to argue about the past, I can do that,” Biden said at the fundraiser. “I got past I’m proud of. They got a past that’s not quite so good.”

Several candidates have prepared for the two-night clash by wooing black voters, a crucial Democratic demographic.

Harris, an ex-prosecutor who has been criticized for opposing some criminal justice reforms while attorney general of California, rolled out a $60 billion plan for black higher education.

Biden unveiled a criminal justice proposal, and Pete Buttigieg, an Indiana mayor in the second tier of candidates, recently presented a plan for what he calls “investment in the empowerment of black America.”

Biden, 76, has acknowledged he was ill-prepared to counter the brickbats against him in the opening debate.

Polls show many black voters are unmoved by the fracas with Biden, who enjoys goodwill from African Americans over his eight-year stint as Barack Obama’s loyal deputy.

A Monmouth University poll released Thursday shows Biden with 51 per cent support among African Americans in early-voting South Carolina.

Booker, currently eighth in polling according to RealClearPolitics, sought to capture some of that support by noting Biden’s new criminal justice plan stands in contrast to the 1994 crime bill Biden helped draft.

Booker dubbed Biden an “architect of mass incarceration” for supporting that bill, and he told the National Urban League that a candidate gets “no great badge of courage” for calling Trump a racist today.

“The question is, what were you doing to address structural inequality and institutional racism throughout your life?”

– Progressive standoff –
Another key showdown on Tuesday will be between the race’s top progressives, Elizabeth Warren and Bernie Sanders.

Both are popular senators advocating for the Democratic Party’s more liberal policies.

Sanders and Warren are essentially tied for second in the race at 15 per cent and 14.5 per cent support respectively. They debated on different nights last month, so Tuesday’s face-off between them could draw fireworks.

Critics warn that progressives could make unforced errors by moving the party positions so far leftward ahead of the 2020 election. Trump is already blasting his Democratic rivals as “socialists.”

Moderate candidates like Senator Amy Klobuchar have expressed scepticism about the viability of Sanders’s “Medicare for All” universal health program and Warren’s plan for tuition-free college.

So has Biden, who has positioned himself as the chief protector of Obamacare, the health law Republicans have repeatedly sought to gut.

The debate will be the first since former special prosecutor Robert Mueller’s congressional testimony last week, which fell short of many Democrats’ expectations that he would dramatically boost the case for Trump’s impeachment.

More than half of the Democratic candidates support impeachment proceedings, and moderators may ask whether such action is more or less likely following Mueller’s underwhelming performance.