Kamala Harris, one of two African-Americans in the race for the Democrat presidential nomination, staked her claim as a leading candidate with an attack on front-runner Joe Biden’s race record.
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Ms Harris, the US senator from California, drew much of the attention in the second round of opening debates when she challenge Mr Biden on his past comments about segregationists.
Mr Biden has faced a swirl of controversy in recent weeks after touting his ability to work with politicians he vehemently disagreed with during his long career in the Senate – including Republicans who supported segregation.
During an emotional exchange, Ms Harris turned to Mr Biden unprompted and said "I do not believe you are a racist" but added that she had found his comments personally "hurtful".
She went on to question why in the past he had opposed "bussing" – the policy of transporting children of different races by buses to break down segregation in schools – and noted that she had personally benefited from the policy as she appeared close to tears.
"That is a mischaracterisation of my position across the board. I did not praise racists. That is not true," Mr Biden said of the attack over his comments about working with Republicans.
He added that he had opposed the policy of busing being adopted by the federal government but backed the right for local authorities to take the move if wanted. Mr Biden also defended his civil rights record during the Obama years.
The exchange – the most heated of the night – exemplified the difficulty Mr Biden faces as he attempts to convince Democrat voters he is the person to reform America in 2020 while defending a political record stretching back almost half a century.
There was a little girl in California who was bussed to school. That little girl was me. #DemDebate pic.twitter.com/XKm2xP1MDH
— Kamala Harris (@KamalaHarris) June 28, 2019
In a bruising night for the former vice president, Mr Biden was told to "pass the torch" of Democratic leadership by a younger rival as he was repeatedly pinned back by other candidates.
Many of the attempts to put the spotlight on Mr Biden, who consistently enjoys a substantial lead over the rest of the pack in opinion polls, appeared to be premeditated.
The 76-year-old pushed back by defending his more moderate positions on healthcare, tax reform and climate change, often pointing to his record as Barack Obama’s deputy for eight years to show he can deliver.
Away from the ‘blue-on-blue’ attacks, there were other moments of unity – not least when all 10 candidates on stage at the Miami debate said their healthcare plans would cover undocumented migrants.
The show of hands prompted a gleeful response from Donald Trump, the US president who managed to follow events despite being in Japan for the G20 summit. He tweeted: "That’s the end of that race!"
But many of the standout moments in the second night of debates between Democrats hoping to take on Mr Trump in the 2020 election involved Mr Biden being challenged by others on the stage.
In one such clash, Eric Swalwell, the 38-year-old US congressman from California, quoted comments Mr Biden made in the 1980s when he first sought the White House.
"I was six years old when a presidential candidate came to the California democratic convention and said it’s time to pass the torch to a new generation of Americans," Mr Swalwell said.
"That candidate was then-senator Joe Biden. Joe Biden was right when he said it was time to pass the torch to a new generation of Americans 32 years ago, he’s still right today."
Mr Swalwell added that the only way to tackle issues like climate change, gun violence and student loan debt was to "pass the torch", deploying the phrase repeatedly. Mr Biden shot back: "I’m still holding on to that torch."
Bernie Sanders, the democratic socialist senator who came second in the party’s 2016 nominee race, also attacked Mr Biden on the issue of Iraq.
As Mr Biden explained away his decision to support the Iraq invasion in 2003, Mr Sanders said: "Joe voted for that war. I helped lead the opposition to that war." Mr Sanders is currently second in most polls behind Mr Biden.
Elsewhere during the two-hour debate, the other candidates attempted to introduce themselves to the millions of Americans watching along on television – many of whom will be unfamiliar with their policy stances.
The other six candidates present were US senators Kirsten Gillibrand and Michael Bennet, South Bend mayor Pete Buttigieg, former Colorado governor John Hickenlooper, entrepreneur Andrew Yang and author Marianne Williamson.
Healthcare proved one issue that split the field. Some candidates like Mr Sanders backed replacing all private health insurance with a public option while others – such as Mr Biden – supported more incremental change, building on Barack Obama’s flagship ‘Obamacare’ legislation.
When candidates were asked by moderators at NBC, the TV broadcaster hosting the event, whether their healthcare plan would cover undocumented migrants all 10 politicians raised their hand.
It led Mr Trump to tweet: "All Democrats just raised their hands for giving millions of illegal aliens unlimited healthcare. How about taking care of American Citizens first!? That’s the end of that race!"
All Democrats just raised their hands for giving millions of illegal aliens unlimited healthcare. How about taking care of American Citizens first!? That’s the end of that race!
— Donald J. Trump (@realDonaldTrump) June 28, 2019
It was his only tweet during the debate. Mr Trump has repeatedly said during campaign rallies that he sees the Democratic position on immigration as a voter winner for his re-election campaign.
Other areas of common ground included rejoining the Paris climate change agreement, ending child migrant detention centres, protecting abortion rights and acting to tackle gun violence.
One question that split candidates was which ally they would call first to improve relations after four years of the Trump presidency. Numerous candidates said the members of Nato, the UN and China. Others named Russia and Iran.
It remains to be seen if the skirmishes dent Mr Biden’s poll lead. Around 30 per cent of Democrats currently would vote for him to be the party’s nominee, according to nationwide opinion polls. The second place candidate tends to be at least 10 percentage points behind.
More debates are to be held in almost every remaining month of this year before the first states begin voting for their nominee in February 2019.
A victor will be crowned at the Democratic national convention in July 2020, ready to take on Mr Trump at the presidential election on November 3.