Heart attacks are not a recommended way of reviving a presidential campaign. But in Bernie Sanders’ case, last October’s coronary surprise came with two silver linings. First, it marked the start of his campaign’s recovery and the fading of Elizabeth Warren’s. Second, it underlined the granite solidity of his base. Not even a heart attack could dampen their faith. Two stents and a record fundraising cycle later, Mr Sanders has good odds of winning the first Democratic caucus in Iowa next month — and New Hampshire the following week.

His stubborn following contrasts with the fluidity of the rest of the Democratic field. No one has much clue of the eventual outcome. Middle Eastern politics is more predictable than this. This time last year, most of the excitement was with Beto O’Rourke and Kamala Harris. Both ran weak campaigns and dropped out. For a few months it looked like Ms Warren had figured out how to be a frontrunner. But her ducking and weaving on healthcare turned out to be an act of enduring self-harm. The Democratic left prizes conviction and authenticity, which are Mr Sanders’ defining qualities.

The 77-year old’s endurance poses two dangers to the Democratic establishment. The first is that Mr Sanders wins the nomination. This is not as far-fetched as it seems. With the exception of Bill Clinton in 1992, no Democrat has ever taken that prize without winning in Iowa or New Hampshire. Seven out of the last nine nominees won Iowa. Every campaign breaks some record so it is possible that Joe Biden could lose both early contests and still go on to take the nomination. But he would be badly wounded. At this point in 2008, Barack Obama was trailing Hillary Clinton in South Carolina. His Iowa victory transformed the race and he went on to win South Carolina. Mr Sanders could pull off the same feat.