DETROIT (CBSDFW.COM/AP) — Democrats battled over health care on Wednesday in the early moments of a presidential debate that tested the strength of Joe Biden’s candidacy.

In particular, the former vice president and one of his chief rivals, California Sen. Kamala Harris, fought over competing plans to provide universal coverage.

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Biden charged that Harris’ plan would cost $3 trillion after she left office and would force middle-class taxes to go up, not down. He said that would put Democrats at a disadvantage against President Donald Trump.

“You can’t beat President Trump with 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.”

The tense exchanges came early on the second night of Democratic debates that pitted the 76-year-old Biden against a younger slate of more diverse candidates. There were no candidates of color onstage in the first wave Tuesday night. On Wednesday night, there were four.

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

Democratic presidential candidate former Vice President Joe Biden (L) speaks while Sen. Kamala Harris (D-CA) listens during the Democratic Presidential Debate at the Fox Theatre July 31, 2019 in Detroit, Michigan. (Photo by Scott Olson/Getty Images)

Biden expects to face pointed questions about race in particular, having stumbled in the opening debate in June when confronted by Harris over his record on school integration. Booker in recent days has seized on Biden’s decades-old support for criminal justice laws that disproportionately hurt minorities.

Biden’s team said that he hopes to focus his attacks on Trump but that he’s ready to fight back more aggressively against Harris and Booker if provoked.

Julian Castro of Texas said he wants to strengthen Medicare for those on it already and give people a chance to buy-in to Medicare.

He supports those with health insurance to keep their plans.

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

In that encounter, Bernie Sanders and Elizabeth Warren faced intense criticism from lesser-known moderates who warned primary voters that a sharp shift to the left on health care and other key policies would make it all but impossible to defeat Trump.

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That same dynamic will be on display Wednesday night — only in reverse.

Biden, who leads virtually all early polls, is considered the premier moderate on stage. 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 Rep. Tulsi Gabbard, New York Sen. Kirsten Gillibrand, Washington Gov. Jay Inslee and entrepreneur Andrew Yang.

Democratic presidential candidates Sen. Michael Bennet (D-CO) (L-R), Sen. Kirsten Gillibrand (D-NY), former housing secretary Julian Castro, Sen. Cory Booker (D-NJ), former Vice President Joe Biden, Sen. Kamala Harris (D-CA) , former tech executive Andrew Yang, Rep. Tulsi Gabbard (D-HI), Washington Gov. Jay Inslee, and New York City Mayor Bill De Blasio take the stage at the Democratic Presidential Debate at the Fox Theatre July 31, 2019 in Detroit, Michigan. (Photo by Justin Sullivan/Getty Images)

Biden and Castro clashed over immigration policy.

Castro has promised to make illegally crossing the U.S.-Mexico border punishable by a civil penalty and is repeating that call Wednesday.

He says that doesn’t mean endorsing “open borders,” but says some in his party, including some on the debate stage, have “taken the bait” and fallen for a Republican talking point.

Biden says he doesn’t support decriminalizing such border crossing. He also says he never heard Castro “talk about any of this while he was secretary.”

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.

The dire stakes have forced many Democrats to turn away from Trump and turn against one another in recent weeks.

Diversity was an early focus.

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, one of Biden’s chief rivals, also referenced the divisive presidency.

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

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(© Copyright 2019 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)