Warren controlled the discussion on implementing a single-payer health care system and also was the foremost candidate advocating for decriminalizing illegal border crossings.

A clear divide emerged between Warren and Sanders on the far left and the more moderate candidates, with South Bend Indiana Mayor Pete Buttigieg and Sen. Amy Klobuchar of Minnesota standing out.

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This divide poses a grave danger for whoever emerges as the Democratic presidential nominee. Unless Democrats can unite and put aside their differences they will face extraordinary difficulties defeating President Trump in November 2020.

Former Rep. John Delaney of Maryland, a dark horse candidate rumored to be considering withdrawing from the presidential race, made a surprisingly strong showing, saying that radical ideas advanced by candidates on the extreme left were impractical and would alienate moderate voters, ensuring Trump's reelection.

The moderates stressed that they were proposing practical solutions to the problems facing the nation that would attract more voters and that would stand a greater chance being approved in Congress.

But both Warren and Sanders said the more incremental solutions proposed by the other contenders were too weak to accomplish what was needed and would not generate the strong voter turnout needed to send President Trump into retirement.

The first issue that came up in the debate, which was broadcast by CNN, was health care. It created some of the most contentious exchanges of the night between "Medicare-for-all" advocates Sanders and Warren and the other more moderate contenders.