Democratic divisions over race, age and ideology burst into public view in yesterday's presidential debate, punctuated by a heated exchange between former Vice-President Joe Biden and California Senator Kamala Harris.

It was one of several moments that left the 76-year-old Biden, who entered the night as his party's early frontrunner, on the defensive as he works to convince voters he's still in touch with the modern Democratic Party — and best-positioned to deny President Donald Trump a second term.

"I do not believe you are a racist," Harris said to Biden before criticising his record of working with Democratic segregationist senators on non-race issues as "hurtful". Biden called Harris' criticism "a complete mischaracterisation of my record." He declared, "I ran because of civil rights" and later accused the Trump Administration of embracing racism.

The night marked an abrupt turning point in a Democratic primary in which candidates have largely tiptoed around each other, focusing instead on their shared desire to beat Trump.

With millions of Americans peeking inside the Democrats' unruly 2020 season for the first time, the showdown revealed deep rifts eight months before primary voting begins.

The showdown featured four of the five strongest candidates — according to early polls, at least. Those are Biden, Senator Bernie Sanders of Vermont, Mayor Pete Buttigieg of South Bend, Indiana, and Harris. Massachusetts Senator Elizabeth Warren, who debated on Thursday, is the fifth.

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 gay.

Harris is the only African American woman to qualify for the presidential debate stage and showed she could land a forceful attack on rivals.

Any of the three women featured yesterday would be the first ever elected president. Yet in the early days of the campaign, two white septuagenarians are leading the polls: Biden and Vermont Sanders.