Its onset has been visible for some time; such is the nature of slow-motion wrecks. Unfortunately we cannot press the fast-forward button. Wednesday night’s debate in Las Vegas was the Democratic party’s nastiest so far — with Michael Bloomberg its clear loser. At some point, nevertheless, the Democratic race is likely to boil down to a fight between him and Bernie Sanders. There are few ways that a zero-sum contest between a self-funded old billionaire and a stubborn old socialist could end in a friendly armistice. There are many in which the collision could play out. Almost none, barring the least likely — a sweeping victory by one over the other — entails a happy ending for Democrats.

The similarities between Mr Sanders’s campaign and Donald Trump’s in 2016 are apt. Each has militant supporters who are happy to indulge in social media harassment and character assassination. Barely half of Mr Sanders’ supporters would vote for Mr Bloomberg if he became the nominee, according to a recent poll. That share would almost certainly rise as the spectre of a Trump second term loomed. But Mr Trump would only need to capture a slice of the “Bernie Bro” constituency — the politically incorrect element of the US senator’s base — to tip the election his way. That is what happened in 2016. An estimated tenth of Mr Sanders’ supporters voted against Hillary Clinton.

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Mr Sanders is trying to do a Trump-like hostile takeover of the Democratic party. He enjoys similar tactical advantages. Like Mr Trump, Mr Sanders has benefited from a packed field of conventional candidates who have spent most of their firepower attacking each other. Wednesday’s debate was the most combustible example of that so far. Like Mr Trump at the same point in 2016, Mr Sanders is spurned by his party’s establishment. Nowadays that is taken as a virtue. A single Iowa legislator endorsed the senator from Vermont before the state’s caucus this month against double-digit endorsements for most of the others, including Joe Biden and Elizabeth Warren. Mr Sanders nevertheless won the most votes.

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