As the only black woman in Democratic White House race, Kamala Harris stole the show at the candidates' first debate by attacking Joe Biden's record on race relations. (Photo: AFP)

The battle for the Democratic White House nomination was thrown wide open Friday after a star debate performance by former prosecutor Kamala Harris, while President Donald Trump licked his chops over the opposition party's sharp leftward turn.

Joe Biden, vice president under still highly popular ex-president Barack Obama, came in as the frontrunner but the 76-year-old stumbled when attacked by younger rivals in the second of two televised debates in Miami, each featuring 10 candidates.

As the only black woman in the 2020 race, Harris already stands out, but she stole the show by attacking Biden's record on race relations, leaving him literally speechless.

Harris, a senator and former attorney general of California, pinned down Biden on his past opposition to school busing programs that forced integration of segregated school systems in the 1970s -- and Biden faltered.

"My time is up," he finally said. Although he'd meant that he had run out of the few seconds granted by moderators on the crowded debate stage, his words went viral on Twitter as a symbolic admission that he is a man of yesteryear in a party demanding radical change.

The radical spirit was on full display both Thursday and in the first debate Wednesday, showing the barriers facing a centrist like Biden on the way to the nomination.

Harris and popular self-described socialist Bernie Sanders joined lesser players in supporting, to various degrees, free healthcare for all, forgiving of student debt, restrictions on private firearms, and an end to Trump's tough crackdown against illegal immigration.

At Wednesday's debate, leftist Senator Elizabeth Warren led the same charge.

While these issues are at the heart of what most agree are the deepest problems in the United States, they are also notorious ideological minefields and never have so many presidential candidates offered such uncompromising solutions.

Trump, on the other side of the world at the Japan G20 summit, gloated.

"All Democrats just raised their hands for giving millions of illegal aliens unlimited healthcare. How about taking care of American Citizens first!?" he tweeted.

"That's the end of that race!"

- Biden wounded or hardened? -

It's too early to say whether Biden, an affable, but gaffe-prone figure, will see his big polling lead melt away.

Trump certainly thinks so.

"Not a good day for Sleepy Joe or Crazy Bernie. One is exhausted, the other is nuts," Trump summarized with his characteristic insults on Twitter.

But the rough treatment, which also included Sanders recalling that Biden had voted for going to war in Iraq, could instead harden the former VP ahead of what will be a brutal battle with Trump.

"While Biden didn't soar, we doubt he was fatally damaged by any of this," veteran politics watcher Larry Sabato said in his post-game analysis.

"Nonetheless, as frontrunner, Biden can look forward to many more attacks. Whether this sharpens Biden for the campaign against Trump (should he win the nomination) or deconstructs Biden on his way to losing the Democratic nod, we cannot guess."

- Harris prosecutes her case -

Despite an impressive resume and compelling personal story as a black woman who rose to the top, Harris, 54, has struggled to gain traction in a field dominated by two elderly white men -- Biden and Sanders -- and also featuring the intense Warren.

Thursday night, she broke free.

In what could only have been a carefully rehearsed attack, drawing on all her experience as a courtroom prosecutor, she first disarmed Biden by saying she was sure he was no racist.

But then she began to question his history on the fraught issue, cornering him with the point that he'd once opposed busing.

In fact, busing was hugely controversial and not all opposition was motivated by wanting racial segregation.

But Harris, her voice quivering with emotion, had an ambush to complete.

"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," she said to the hushed audience -- and to Biden.

"That little girl was me."

While the Florida debate was still going, the Harris campaign tweeted out that killer line with a picture of Harris as a schoolgirl in pigtails. A T-shirt bearing the same image went on sale immediately after.

- Taking on Trump -

So many Democrats are struggling to take on Trump that they are likely to fight each other for a long time first.

At the first debate on Wednesday, Trump's name was hardly mentioned.

That changed on Thursday, with candidates rounding on Trump's criminalization of illegal immigrants, and what they said were his policies favoring the rich over the middle classes -- even if Trump boasts that the roaring US economy benefits all Americans.

"Trump is a pathological liar and a racist," Sanders blasted.

Harris called Trump "the greatest national security threat to the United States" because of his rejection of warnings that the climate is in the midst of disastrous change.

Biden, seeking a recovery in his closing remarks, called for restoration of "the "soul" of the country after Trump "ripped it out."

"We have got to unite the United States of America," he said.