By Anthony Zurcher BBC North America reporter
There have been seven official Democratic debates but this one had a real sense of urgency – in less than three weeks the candidates will face their first test.
The primary season begins on 3 February with the Iowa caucuses, when the Democratic voters in this state will pick who they want to take on Donald Trump in November.
As the six White House hopefuls took to the debate stage in Des Moines, the Republican US president they have in their sights was mocking them at a rally 400 miles east in Wisconsin.
Here are some key moments from the debate – and the Trump rally.
The body language between Elizabeth Warren and Bernie Sanders at the conclusion of the debate – when they spoke heatedly and did not shake hands – told its own story.
The non-aggression pact between the two candidates could be over just as the voting is set to begin.
The biggest news story in campaign politics over the past few days had been the growing tensions between the two most liberal candidates.