The leading candidates for the Democratic presidential nomination have clashed in their first appearance together in the race to take on Donald Trump next year.

Former vice-president Joe Biden, who polls show remains as the leading contender, faced off against his two closest rivals, senators Bernie Sanders and Elizabeth Warren.

And the dispute over how to tackle the perennial problem of America’s healthcare system once again exposed the divisions in the Democrat field between progressives and moderates.

Mr Biden, who served eight years under Barack Obama, has promised to build on the Obamacare policy which Donald Trump has attempted to dismantle. He accused Ms Warren and Mr Sanders of wanting to tear it down.

But Ms Warren says a bigger plan is needed and wants the country’s top earners to pay more in taxes to fund a government-run health plan for everyone in the country.

Mr Sanders shares her views on what’s known as ‘Medicare-for-all’.

In a heated back-and-forth, Mr Biden said: This is about candour, honesty, big ideas.

The candidates tried to present a more collegiate atmosphere than in previous debates but former housing secretary Julian Castro accused Biden of flip-flopping on healthcare.

Are you forgetting what you said two minutes ago? he asked Mr Biden, who has been prone to memory slips and gaffes in recent weeks.

Before the debate began in Houston, a banner promoting Donald Trump’s re-election campaign was flown over the venue by his supporters. Mr Trump had earlier said he might watch a re-run of the debate later.

Inside the auditorium, opposition to the Republican president allowed the candidates a show of welcome unity for Democrats.

There was praise for former congressman Beto O’Rourke’s response to the mass shooting in his home town of El Paso.

He repeated his call for a ban on assault weapons: Hell yes, we’re going to take your AR-15, your Ak-47.

On gun violence, immigration, racial inequality, climate change and foreign affairs, all of the candidates promised a radically different approach to the current occupant of the White House.

So far these debates have done little to shift the needle in this race.

Opinion polls have shown it to be a contest that is Biden’s to lose.

In Houston, the candidate who would be the oldest person ever to take office as president, at times appeared flustered and frustrated with the attacks on his record.

In a surprising detour on education in America, he advised parents to make sure the record player is on at night, make sure kids hear words.

But many see Mr Biden’s appeal in blue-collar states that Mr Trump won in 2016 – states like Pennsylvania, Wisconsin and Ohio – as being the single most important factor in who they will chose as their candidate.

Impressive performances by Warren, senators Kamala Harris and Cory Booker, and ‘Mayor Pete’ Buttigieg are certain to bring about a shift in the polling order below Biden.

Harris provided one of the night’s more memorable quotes on Mr Trump’s trade policy: He reminds me of that guy in The Wizard of Oz. You know, when you pull back the curtain, it’s really small dude.

And entrepreneur Andrew Yang’s offer of a $1000 a month freedom dividend to 10 Americans for the next year was surely one of the most surprising debate gambits in history.

It’s original. I’ll give you that, said Mr Buttigieg.

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(c) Sky News 2019: Democratic presidential candidate front-runners divided over healthcare