Image copyright Getty Images Image caption Elizabeth Warren (L), Bernie Sanders (C) and Julián Castro (R) are all contenders for the Democratic presidential nomination

Democratic presidential candidates have given their reaction to a warning by former President Barack Obama against moving too far left in politics.

Mr Obama's rare intervention into the Democratic race was a talking point at campaign events on Saturday.

Some Democrats called for unity, while others defended their policy agenda.

Nearly 20 candidates remain in the running and there is much debate over the best approach to taking on President Trump next year.

Speaking at a fundraising forum in Washington, the former president - considered a moderate - cautioned candidates against pursuing polices that were not "rooted in reality".

Mr Obama, who was in office from 2009 to 2017, said "ordinary Americans" didn't want to "completely tear down the system".

"This is still a country that is less revolutionary than it is interested in improvement," Mr Obama said to an audience of wealthy donors on Friday.

Media playback is unsupported on your device Media caption Watch former US President Barack Obama talk about "woke" culture

The remarks represented Mr Obama's most pointed intervention yet in a crowded race featuring 18 candidates.

Former vice-president Joe Biden and senators Elizabeth Warren and Bernie Sanders are leading the pack, but Mr Obama is yet to publicly back a candidate.

How did candidates respond to Mr Obama?

Although none of the Democratic candidates explicitly rebuked Mr Obama's comments, Mr Sanders mounted the strongest defence of his policy platform.

Answering questions on a forum aired by Univision, a Spanish-language TV network, he was asked whether Mr Obama was "right" to say voters didn't want systemic change.

Mr Sanders, who describes himself as a democratic socialist and progressive, laughed and said: "Well, it depends on what you mean by tear down the system."

Image copyright Getty Images Image caption Vermont Senator Bernie Sanders insisted that he was "fighting for justice", not seeking to tear down the system

"The agenda that we have is an agenda supported by the vast majority of working people," he said. "When I talk about raising the minimum wage to a living wage, I'm not tearing down the system. We're fighting for justice."

Elizabeth Warren, another left-leaning frontrunner, struck a more conciliatory tone, choosing to praise Mr Obama's trademark health care policy, the Affordable Care Act.

"I so admire what President Obama did," Ms Warren said at a campaign event in Iowa, the New York Times reported.

"He is the one who led the way on health care and got health care coverage for tens of millions of Americans when nobody thought that was possible."

Image copyright Getty Images Image caption Elizabeth Warren said she admired Barack Obama's health care achievements

New Jersey Senator Cory Booker said the party ought to be focusing its energy on defeating Republican President Donald Trump, not internal political squabbles.

"Let's stop tearing each other down, let's stop drawing artificial lines," he said.

Unlike Mr Obama, Julián Castro, a former mayor of San Antonio, Texas, said he was confident any Democratic candidate would beat President Trump, regardless of their political persuasion.

Image copyright Getty Images Image caption Julián Castro said he was confident any Democratic candidate would beat Donald Trump

"Their vision for the future of the country is much better and will be more popular than Donald Trump's," Mr Castro, former housing secretary in the Obama administration, said.

Obama tips his hand

Obama has studiously avoided weighing in on the large field of Democratic candidates vying for the party's 2020 presidential nomination. Behind closed doors on Friday, however, he tipped his hand a bit.

Sanders is preaching political revolution. Warren is urging "big systemic change". The former president clearly had those two frontrunners in mind when he suggested such aggressive talk risks alienating the kind of middle-of-the-road voters necessary to defeat Donald Trump next year.

This shouldn't come as too much of a surprise. Obama, despite being labelled a radical socialist by his conservative critics, governed as a pragmatic moderate. That created a fair amount of consternation of among progressives in his party, who thought he was one of their own when elected. Some view his presidency as a missed opportunity to enact fundamental structural reforms in the wake of the 2008 economic crisis.

This time around, they're throwing their support behind Warren and Sanders and won't appreciate being indirectly lectured by the former president.

The moderate-progressive division within the Democratic Party is very real, and it has the potential for combustion. Obama may not be picking a favourite candidate, but it looks like he's picking sides.

Others not involved in the race for the nomination were more blunt.

In a tweet, Peter Daou, a former aide to Hillary Clinton, wrote: "Saying 'Americans are moderate than these wild leftists' is basically conceding that the far-right propaganda machine has prevailed."

In a later tweet, Mr Daou included the hashtag #TooFarLeft, which was widely used by other social media users who disagreed with Mr Obama.

The Democratic race is still in flux as the first of the state-by-state votes that will decide which of the contenders challenges Mr Trump for the White House looms in Iowa in February.

Pete Buttigieg, mayor of South Bend, Indiana, topped the latest poll of likely Democratic voters in Iowa.

Some Democrats are concerned that Mr Biden, a moderate who was vice-president to Mr Obama, will struggle to beat Mr Trump, prompting a flurry of latecomers to join the race.

Image copyright Getty Images Image caption Barack Obama said most "ordinary Americans" didn't want to completely tear down the system

In recent days Deval Patrick, the two-time former governor of Massachusetts, entered the field amid speculation that former New York Mayor Michael Bloomberg may follow suit.

But Democratic hopes of electoral success in 2020 were boosted on Saturday after Democratic Governor John Bel Edwards secured a second term as Louisiana governor.

Who will take on Trump in 2020?

Election day is less than a year away now and the race to become the Democratic challenger to Donald Trump is heating up.

The latest polling suggests Joe Biden and Elizabeth Warren are the front-runners, while Bernie Sanders and Pete Buttigieg are not far behind.