DETROIT: The ideological divisions gripping the Democratic Party intensified on Wednesday as presidential candidates waged an acrimonious battle over health care, immigration and race.

The second presidential debate tested the strength of early front-runner Joe Biden, who again was forced to defend his decades-old political record on multiple fronts as other White House hopefuls sought to tear him down.

One of his chief rivals, California Senator Kamala Harris, charged that Biden’s past work with segregationists in the US Senate could have prevented Barack Obama from becoming the nation’s first black president.

“Had those segregationists had their way, I would not be a member of the United States Senate, Cory Booker would not be a member of the United States Senate, and Barack Obama would not have been in a position to nominate” Biden to become vice president.

In response to a separate attack from Booker, Biden leaned into his relationship with Obama.

“We’re talking about things that occurred a long, long time ago,” Biden said. He added: “Everybody’s talking about how terrible I am on these issues. Barack Obama knew who I was.”

The conversation underscores an escalating rift over race shaping the Democratic primary just two weeks after President Donald Trump issued racist calls for four female congresswomen of colour to leave the country, even though all of them are American citizens. Over the weekend, Trump again took aim at a prominent congressman of colour, charging that “no human being would want to live” in his “rat-infested” Baltimore district, which has a large black community.

This is an internal fight many Democrats do not want. For Biden’s struggling 2020 competitors, however, they see no better way to undermine his candidacy than raising questions about his commitment to black voters.

Wednesday’s debate comes 24 hours after another set of 10 Democrats debated, fiercely at times, over the direction of their party.

Biden, who leads virtually all early polls, is considered the premier moderate onstage. In addition to Harris and Booker, his more progressive opponents include New York Mayor Bill de Blasio, former Obama administration housing chief Julián Castro, Hawaii Representative Tulsi Gabbard, New York Senator Kirsten Gillibrand, Washington Governor Jay Inslee and entrepreneur Andrew Yang.

Biden charged that Harris’ health care plan would cost taxpayers $3 trillion even after two terms in office and would force middle-class taxes to go up, not down. He said that would put Democrats at a disadvantage against Trump.

“You can’t beat President Trump with a double talk on this plan,” he said.

Harris slapped back that Biden was inaccurate.

“The cost of doing nothing is far too expensive,” Harris said. She added: “Your plan does not cover everyone in America.”

There were also tense exchanges on immigration that pitted the 76-year-old Biden against a younger slate of more diverse candidates. There were no candidates of colour onstage in the first wave Tuesday night. On Wednesday night, there were four.

Biden was flanked by Harris on one side and Booker on the other. As Biden greeted Harris onstage moments before the opening statements, he quipped, “Go easy on me, kid.”

Biden suggested that some of his rivals favour immigration laws that are far too forgiving. Castro, for example, would decriminalize illegal border crossings.

“People should have to get in line. That’s the problem,” Biden charged.

Castro shot back: “It looks like one of us has learned the lessons of the past and one has not.”

While the first primary votes won’t come for six more months, there is a sense of urgency for the lower-tier candidates to break out. More than half the field could be blocked from the next round of debates altogether — and possibly pushed out of the race — if they fail to reach new polling and fundraising thresholds implemented by the Democratic National Committee.

Biden said Trump was tearing at the “fabric of America” and highlighted the value of diversity in his opening statement.

“Mr President, this is America,” Biden said of the diverse slate of candidates on stage.

Harris also referenced Trump’s divisive presidency.

“This becomes a moment we must fight for the best of who we are,” Harris said. “We are better than this.”