MIAMI — Sen. Kamala Harris of California sharply challenged former Vice President Joe Biden on his record on race relations Thursday night, taking on the front-runner in an emotionally charged exchange during the second of two Democratic presidential debates.

“I do not believe you are a racist,” Harris, a former prosecutor, began coolly before turning to Biden’s recent comments about finding cooperation with segregationist senators in the 1970s. “It’s personal and was hurtful to hear you talk about two United States senators who built their reputation and career on segregation of race in this country.”

Biden, by far the front-runner of the large Democratic field in polling, has had fewer public events and taken fewer public questions than many of the other contenders, and while they had been cautious in their criticism of the former vice president, Harris waded right in.

“A complete mischaracterization — I did not praise racists, that is not true,” Biden responded, pointing to his long history of supporting civil rights laws, his tenure as a public defender rather than as a prosecutor, and his work alongside former President Barack Obama. “I ran [for office] because of civil rights.”

Biden hit on one potential vulnerability for Harris: her record as a tough prosecutor and attorney general in California in a time of concern about mass incarceration, particularly of African Americans.

Some of Biden’s four-decade record also has seemed out of touch with the Democratic Party’s center of gravity. Amplifying her point, Harris turned to Biden’s past opposition to school busing to aid integration, touching on a potentially explosive topic.

“There was a little girl in California who was part of the second class to integrate her public schools, and she was bused to school every day, and that little girl was me,” said Harris, the daughter of Indian and Jamaican parents.

Biden said he only opposed federally mandated busing, not when it was a state and local decision. He reminded Harris that the busing program she was involved in "was a local decision made by your city council.”

Harris refused to let go. “There are moments in history where states fail to support” civil rights, she said.

The exchange at the Adrienne Arsht Center for the Performing Arts was the most pointed attack on Biden, who has led the race since joining it in April, in part because of his strong support among African Americans. But he has faced questions about his staying power and readiness for direct confrontation, especially after keeping a light public schedule.

Harris, also one of the top five contenders according to polling, dominated key moments. By the end of the night, Google tweeted that Harris was the top trending search in the United States as she blended policy views, measured attacks, personal stories and sharply delivered quotes.

“America does not want to witness a food fight, they want to put food on their table,” she said during one hectic moment of arguments.

She also challenged Biden on immigration policies during the Obama administration, saying it was too aggressively focused on deporting undocumented immigrants.

The exchanges on race came after these 10 of the Democrats running for president clashed over how to advance one of the party’s enduring aims: expanding health insurance coverage so everyone has it.

The issue consumed the early phases of a debate featuring four of the top five candidates in polling for the 2020 race, with Sens. Bernie Sanders (Ind., Vt). and Harris arguing for doing away with private health coverage, while opponents such as Biden and South Bend, Ind. Mayor Pete Buttigieg called for a slower approach that would allow people to keep their private insurance plans if they chose.

“The function of the health care system today is to make billions in profits for the insurance companies,” Sanders said, “while we pay the highest prices in the world for prescription drugs.”

Sanders, whose “Medicare for All” plan has become the fulcrum of the Democratic argument on health care, conceded that middle-class Americans might pay more in taxes under his array of proposals, but said they would save in what they pay for health care.

Harris joined him in calling for replacing private insurance with a single-payer government run plan, such as Medicare.

Contrasting with Sanders’ calls for political revolution and sweeping change, she instead delivered an example of a mother taking her sick child to an emergency room, “knowing that if they walk through those sliding glass doors, even though they have insurance, they will be out ... a $5,000 deductible.”

Other Democrats cast such drastic change as unwieldy and unrealistic.

Every Medicare for All supporter “has a responsibility to explain how you’re actually supposed to get from here to there,” Buttigieg said. He argued that giving people Medicare as a voluntary option “will be a very natural glide path to a single-payer environment.”

Biden again pointed to his work with Obama and the Affordable Care Act, one of the most significant Democratic policy gains in recent memory.

“The quickest, fastest way to do it is to build on Obamacare, to build on what we did,” Biden said.

While Biden might have absorbed the toughest blows, Sanders faded once the debate moved beyond health care, and several other candidates struggled to make their marks, despite trying to jump into the major exchanges.

The debate came with weighty expectations, with Biden, Sanders, Harris and Buttigieg on the stage together. The only leading contender missing was Sen. Elizabeth Warren of Massachusetts, who had driven the discussion on economics Wednesday night. A random drawing decided which candidates appeared each night.

Together, the debates Wednesday and Thursday gave a national audience a look at 20 of the 25 Democrats running, and gave the candidates their first chances to shape the race after months of introductions and smaller events.

Immigration dominated another significant portion of the debate, with multiple candidates calling for more humane treatment of people seeking asylum and those detained at the border, and saying they would reinstate the program that protected people brought to the country illegally as young children.

“We should call hypocrisy on a party that associates with Christianity to say it is OK to suggest that God would smile on the division of families at the hands of federal agents,” Buttigieg said.

Sanders said simply he would “rescind every damn thing on this issue that Trump has done."

Also on stage were Sen. Michael Bennet of Colorado, Sen. Kirsten Gillibrand of New York, former Gov. John Hickenlooper of Colorado, author Marianne Williamson, and Andrew Yang, an entrepreneur.