"Elizabeth is right," Sanders said at one point, summing up the tone of the night on Tuesday, US time. Their fight wasn't with each other but with Democrats who say the party has to adopt a cautious, centrist policy platform if they want to beat Donald Trump in 2020. Friends and fierce opposing candidates: Bernie Sanders and Elizabeth Warren. Credit:AP Sanders, the 77-year-old Democratic socialist from Vermont, put in a punchier, more impressive performance than in the last round of debates. But he was outshone by Warren, who easily won the night and cemented her status as a leading candidate for the party's nomination.

Since announcing her candidacy Warren has shown she can produce detailed and innovative policy proposals. She also showed she can deliver something else Democrats want in their candidate: a forceful debater who can go in for the kill against an opponent. Warren delivered the best line of the debate when responding to former congressman John Delaney, who warned against the party moving too far to the left. Elizabeth Warren supporters turn up for the debate in Detroit, Michigan. Credit:Bloomberg "I don’t understand why anybody goes to all the trouble of running to be the president of the United States to talk about what we really can’t do and shouldn’t fight for,” Warren said. "I don’t get it."

Her riposte drew big applause from the crowd, and rammed home the message that she was the candidate with the courage to fight for her beliefs. Loading Warren made the case that Democrats don't have to trade away bold ideas in order to win. She gave them permission to feel excited rather than afraid. "There is a lot at stake, and people are scared," she said. "But we can’t choose a candidate we don’t believe in just because we’re too scared to do anything else. And we can’t ask other people to vote for a candidate we don’t believe in." The more moderate candidates on the stage - like Delaney and Montana Governor Steve Bullock - tried to point out the flaws in Warren and Sanders' approach.

Democrat presidential hopefuls line up for Tuesday's debate in Detroit. Credit:AP They were right to point out that tens of millions of Americans - including union members - do not want to lose their private health insurance and would be alarmed by the idea of shifting to a government-run system. They were right to point out that Warren's stance on immigration - that it should be legal to enter the US without permission - is unnecessarily radical and politically dangerous. The problem is that they sounded more like political commentators than candidates. They were able to pick holes in the progressives' arguments but couldn't cut through with their own ideas. That's why they are all wallowing in single digits in the polls.