MIAMI — Democratic divisions over race, age and ideology surged into public view Thursday night as the party's leading presidential contenders faced off in a fiery debate over who is best positioned to take on President Donald Trump.

The Democratic Party's early front-runner, 76-year-old former Vice President Joe Biden, was forced to defend his record on race in the face of tough questions from California Sen. Kamala Harris, the only African American on stage. That was only after he defended his age after jabs from one of two millennial candidates in the prime-time clash.

"I do not believe you are a racist," Harris said, though she described Biden's record of working with Republican segregationist senators on non-race issues as "hurtful."

Clearly on defense, Biden called Harris attack "a complete mischaracterization of my record." He declared, "I ran because of civil rights."

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For moments Thursday night, a fierce intraparty debate that had been simmering just below the surface about the party's future was exposed on national television. The showdown featured four of the five strongest candidates — according to early polls, at least. Those are Biden, Sanders, Pete Buttigieg of Indiana and Harris. Massachusetts Sen. Elizabeth Warren, who debated Wednesday night, is the fifth.

Together, the two nights gave millions of Americans their first peek inside the Democrats' unruly 2020 season.

There are so many candidates lining up to take on Trump that they do not all fit on one debate stage — or even two. Twenty Democrats debated on national television this week in two waves of 10, while a handful more were left out altogether.

The level of diversity on display was unprecedented for a major political party in the United States. The field features six women, two African Americans, one Asian American and two men under 40, one of them openly gay.

Yet in the early days of the campaign, two white septuagenarians are leading the polls: Biden and Vermont Sen. Sanders.

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Sanders early on slapped back at his party's centrist candidates, demanding "real change." Biden downplayed his establishment leanings.

For example, the former vice president, along with the other candidates on stage, raised his hand to say his health care plan would provide coverage for immigrants in the country illegally.

Sanders' appeal relies on emotion, often anger. He stood alongside Biden, who preaches pragmatism and relative moderation.

Biden, like Sanders, who is 77, also represents a different generation from several candidates on stage. The age difference was noted by California Rep. Eric Swalwell, who said, "Joe Biden was right when he said it was time to pass the torch to a new generation of Americans 32 years ago."

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Others on the stage Thursday night included South Bend, Indiana, Mayor Buttigieg and Harris, who have shown support in opinion polls. Also: Sens. Kirsten Gillibrand of New York and Michael Bennet of Colorado, former Colorado Gov. John Hickenlooper, New York businessman Andrew Yang, California Rep. Swalwell and author and social activist Marianne Williamson.

The showdown played out in Florida, a general election battleground that could well determine whether Trump wins a second term next year.

Biden sought to sidestep the ideological debate altogether, training his venom on Trump.

"Donald Trump thinks Wall Street built America. Ordinary middle-class Americans built America," said the former vice president. He added: "Donald Trump has put us in a horrible situation. We do have enormous income inequality."

Biden's strategy is designed to highlight his status as the front-runner, and as such, the Democrat best positioned to take down the president at the ballot box. Above any policy disagreement, Democratic voters report that nothing matters more than finding a candidate who can beat Trump.

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If nothing else, Thursday's slate highlights the diversity of the Democratic Party's 2020 class.

Buttigieg, a 37-year-old gay former military officer, is four decades younger than Sanders, and has been framing his candidacy as a call for generational change in his party. Harris is the only African American woman to qualify for the presidential debate stage. Any of the three women featured Thursday night would be the first ever elected president.

Yet Biden and Sanders have received far more attention and shown higher standing than their less-experienced rivals.

The party will have to decide whether it wants a candidate based on resume over aspiration.

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Juana Summers reported from Miami. Steve Peoples reported from Washington.