The Democratic debates’ standouts have been Booker, Buttigieg, Castro, Harris and Warren – none a straight male Caucasian – while Joe Biden looked like this year’s Jeb Bush

It was as if “Sleepy Joe” Biden had been jolted awake and found himself in a terrifying parallel universe.

No longer was he the former senator and vice-president who once lunched weekly with Barack Obama at the White House. Instead he was the accused, standing in the dock under the hot lights on a debate stage in Miami, as a fierce and formidable prosecutor tore into his record on race.

The inquisitor was Senator Kamala Harris, a black and Asian American former prosecutor and California attorney general and who has shown a rare talent for making Jeff Sessions and other ageing white men in power squirm. Harris was surely the stand-out performer of the second in a pair of primary debates this week where the world could see a generational shift in the Democratic party happening before its eyes.

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In the most dramatic moment, Harris challenged former Biden over his recent comments about working with segregationist senators early in his career, as well as opposing government-mandated bussing – transporting students to schools within or outside their local districts to rectify racial segregation – in the 1970s. It was an exchange that illustrated a yawning gap in age and race.

“I do not believe you are a racist,” she said in a withering tone for which she is becoming famous.

“And I agree with you when you commit yourself to the importance of finding common ground. But I also believe – and it’s personal and it was hurtful to hear you talk about the reputations of two United States senators who built their reputations and career on the segregation of race in this country.”

The senator pressed on: “It was not only that, but you also worked with them to oppose bussing. There was a little girl in California who was part of the second class to integrate her public schools, and she was bussed to school every day.”

Then, the killer line: “That little girl was me.”

Kamala Harris (@KamalaHarris) There was a little girl in California who was bussed to school. That little girl was me. #DemDebate pic.twitter.com/XKm2xP1MDH

The crowd erupted. It was the moment in every courtroom drama when the accused is skewered. It was the mic drop moment. It could possibly end up being the Biden-loses-his-third-bid-for-president moment.

Looking rattled, Biden insisted that Harris “mischaracterised my position across the board. I did not praise racists. It is not true, number one. Number two, if you want to have this campaign litigated on who supports civil rights and whether I did or not, I’m happy to do that. I was a public defender. I didn’t become a prosecutor.”

It was an attempted swipe back at Harris, whose career as a prosecutor has drawn scrutiny from progressives. Biden added: “The bottom line here is: look, everything I have done in my career – I ran because of civil rights.”

Quick guide Who are the leading Democrats running for 2020? Show Hide Joe Biden, former vice president Biden unsuccessfully ran for the nomination in 1988 and 2008, and his campaign is likely to be dogged by controversy after allegations from several women they were left feeling uncomfortable by their physical interactions with him. If successful, Biden would become the oldest person to be elected president in US history. Mike Bloomberg, former New York mayor Bloomberg has expressed concern that none of the top candidates can defeat Trump, and he aims to make up for an unusually late entry in the Democratic primary with historic spending of hundreds of millions of dollars in television ad time and an unorthodox strategy of skipping the first four states in the primary. Bloomberg has announced that his campaign will be entirely self-funded, but can this billionaire win? Amy Klobuchar, Minnesota senator On Election Night 2018, Klobuchar coasted to a third term as senator in a state Trump almost won. Next morning she was on every short list of potential presidential candidates. Supporters say her success with rural voters makes her a formidable candidate in the Rust Belt, while her calm demeanour provides a clear contrast with Trump. Bernie Sanders, Vermont senator Sanders turned a long-shot, anti-establishment bid for the presidency into a “political revolution” that energized the party’s progressive base. His political career began nearly 40 years ago, but it wasn’t until his 2016 run that Sanders became a national figure as a new generation of Democrats – and 2020 contenders – embraced his populist economic policies. Elizabeth Warren, Massachusetts senator Her sharp criticism of Wall Street and big corporations has made Warren a favorite among progressive activists, and she will campaign on a message of a rigged economic system and income inequality.

But Harris, evidently aware that making a splash in these debates requires made-for-TV confrontation, had the accused on the ropes. “Vice-President Biden, do you agree today that you were wrong to oppose bussing in America?”

Biden retorted: “I did not oppose busing in America. What I opposed is bussing ordered by the Department of Education [rather than leaving the decision to states]. That’s what I opposed.”

Harris shot back: “There are moments in history where states fail to support the civil rights of people.”

It was a bruising exchange that exemplified the two-night debate involving 20 candidates, the speaking of Spanish and the championing of trans rights. The breakout stars were Cory Booker, Pete Buttigieg, Julián Castro, Kamala Harris and Elizabeth Warren. None is a straight white male. A changing of the guard.

Indeed, Thursday’s debate put Biden and Bernie Sanders centre-stage, two white men with a combined age of 153. On either side of them were Harris, 54, and Buttigieg, the mayor of South Bend, Indiana, who at 37 is youngest in the field and bidding to become the first openly gay president. It was Harris and Buttigieg, who was honest about his failings regarding policing and race relations, who carried the day.

Another young upstart, Eric Swalwell, made a clumsy rhetorical lunge at Biden early on, but was on to something when he said: “Joe Biden was right when he said it was time to pass the torch to a new generation of Americans 32 years ago.”

The vice-president did little to dispel that notion. Along with his grilling by Harris, he appeared rambling and uninspiring. When the candidates were asked what their first priority as president would be, he mysteriously answered beating Donald Trump. Biden seems determined to be this year’s Jeb Bush.

Sanders hit the usual notes for his base but did little to expand it. Asked about diversity, he struggled, as always, and reverted to economics. He too seemed out of touch with the America of Black Lives Matter.

Naturally, Donald Trump came in for some stick, with Sanders describing him as “a pathological liar and a racist”.

There were those in the real debate and then there was the bizarro debate. Andrew Yang, a former tech entrepreneur, and Marianne Williamson, an author, proved how hard it is for ordinary civilians to make an impression when you’re not a celebrity billionaire with a machine gun of insults.

Play Video 3:27 US election 2020: highlights from second night of Democratic debates – video

At times there was a cacophony of voices with everyone trying to get a word in. Again, it was Harris who won that round by interjecting: “America does not want to witness a food fight. They want to see how we’re going to put food on their table.”

She emerged as the victor of not only Thursday but Wednesday too. Last week, at a town hall with the Poor People’s Campaign in Washington, she seemed somewhat vague and a little pandering.

But on a debate stage, or in a committee room on Capitol Hill, she is in her element. Faced with presidential debates against 73-year-old white male Donald Trump if she is nominated as the Democratic party’s candidate, perhaps that is what matters most.

One other question lingered in the air. What did Obama make of it all?

• This article was amended on 1 July 2019 to correct a quote from Kamala Harris, who said “America does not want to witness a food fight”, not “… a food night” as an earlier version said.