The biggest, most electrifying event in the closing days of campaigning in New Hampshire was not staged by Bernie Sanders or Pete Buttigieg or even the surprise third-place finisher, Amy Klobuchar. No, the candidate who filled a 12,000-seat arena, and had devotees queueing up for several blocks on a frigid Monday evening, was the man those others are battling to take on in November, the man who is shaping the Democratic presidential race and who is inside the heads of Democratic voters: Donald Trump.

The commitment of those Trump supporters, the intensity of their faith, was on a scale unmatched by anything else I saw as I crisscrossed New Hampshire. The loyalty inspired by Sanders comes closest, but it’s in a distant league from the adoration offered to Trump. Those lining up in the snow to see the president, buying up hats and T-shirts bearing his name or face, spoke freely of their “love” for him, of their certainty that every criticism was fabricated by “the fake-news media”, of their conviction that he is the only teller of truths in a world of lies.

Trump’s presence in New Hampshire on the eve of the first formal vote of the primary season was a clarifying reminder of what Democrats are up against. Trump has amassed a vast war chest; his re-election effort is well organised, ready to exploit all the advantages of incumbency; the US economic picture is rosy, essential for any president seeking a second term; and last week he notched up his highest approval rating since taking office.

But if the scale of the challenge is clear, so too is the necessity of the task. Since last week’s acquittal in his impeachment trial, Trump has acted like a ruler unbound, whether purging career officials who dared testify against him or meddling in the justice system to secure a more lenient sentence for a pal, Roger Stone, over crimes Stone committed to protect Trump. The president seems to have read the Senate verdict as proof that he is above the law, able to act with impunity. Democrats scarcely exaggerate when they warn that four more years of Trump endangers the country’s status as a constitutional republic.

The story of the 2020 race so far is the failure of the non-Sanders majority to agree on a single standard-bearer

Small wonder, then, that the central question of the contest just completed in New Hampshire was: who can beat Trump? Witness the exit poll that found Democratic voters were looking for a candidate who can win in November, as opposed to one who shares their view of the issues, by a margin of two to one. The trouble is, they can’t agree on who that person is – or even what qualities they will need.

Current polls that show all the main candidates defeating Trump are little use because they can’t anticipate how those candidates would fare after months on the receiving end of a Trump bombardment. Naturally, Sanders says he is best placed to win. His supporters wear badges promising that “Bernie beats Trump”, a case buttressed by winning the most votes in both New Hampshire and Iowa. Despite a heart attack in October, he remains a vital presence on the stump and is, if anything, an even stronger, more focused speaker now than he was four years ago.

And yet in 2016 he beat Hillary Clinton in New Hampshire by 22 points; this time, in the backyard of his home state, his margin over Buttigieg was just 1.5%, a matter of 4,000 votes. True, this was a much more crowded field than in 2016, but the crowding was in the “moderate” camp rather than in Sanders’ terrain. Wherever you went in New Hampshire, you heard the same concerns, even from those who liked Sanders’ radicalism: that he was just too “extreme” to get elected. Democrats can anticipate the attack that Trump will unleash, branding Sanders a “crazy socialist”.

Indeed, Trump’s supporters have already absorbed that message. One man waiting to get into the Monday rally told me the Vermont senator was not American but “Russian, I’m sure. He’s a socialist. He got married in Moscow.”

In fact, Sanders spent his honeymoon in the Soviet Union in 1988, but these are niceties that will not detain Trump, and Democrats know it. Some strategists fear that if Sanders is the nominee, the party will have to write off the vital swing state of Florida – home to many voters with roots in South America, for whom the word “socialist” is a reminder of the regimes they fled. Or if it is not yet a reminder, it will be once Trump is done.

Those anxieties about electability help explain why Sanders, though now leading the Democratic pack, is hovering around the 25% mark, unfavoured by three in four Democrats. And yet the story of the 2020 race so far is the failure of the non-Sanders majority to agree on a single standard-bearer, thereby splitting the pragmatic vote, giving Sanders the chance to keep winning – albeit on a relatively modest vote share – so that he steadily picks up enough delegates to secure the nomination.

And why is it proving so hard for non-Sanders Democrats to unite behind an alternative? Because of that same worry about electability and, specifically, what Trump would do to any one of those candidates come the autumn. After the trauma of 2016, Democrats ascribe an almost magical power to Trump and his ability to define an opponent negatively in a way that strikes a deep chord in the American psyche. Among voters I met in New Hampshire, it became paralysing: each time they imagined a candidate up against Trump, they recoiled in fear at the beating the president would probably inflict.

That’s one reason why Joe Biden and Elizabeth Warren did so badly. Voters could see how Trump would monster Biden as both past his prime and tainted by his son’s lucrative role in Ukraine. And they didn’t need to guess at how he would deal with Warren, tapping into the same misogynistic discomfort with an older, opinionated woman that proved so damaging to Clinton. Indeed, one of the saddest experiences in New Hampshire was speaking to women voters who desperately wanted to see a female president – but who had glumly concluded that the country was still not “ready”.

So, left to their own devices, Democrats might like Buttigieg – though many worry that neither he nor Klobuchar can connect with the voters of colour who are so central to the Democratic coalition and who will be decisive in the coming contests in Nevada and South Carolina. But they can also picture Trump mocking Buttigieg as a small-town mayor, at 38 too young and inexperienced for high office – a high school debating champion, not a president. And they suspect that Trump will find a way, through his darkest arts, to make Americans uncomfortable at the prospect of a gay man in the White House.

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It is telling that one of Klobuchar’s applause lines comes when she urges her audience to imagine her on the debate stage alongside Trump. Her case is that, unlike her rivals, she does not carry the baggage, or obvious weakness, that the president can exploit. Others reckon Trump would find a flaw in Klobuchar eventually, with misogyny his obvious weapon of choice. (Those who know the senator warn that behind the veneer of “Minnesota nice” is a character more abrasive than appearances might suggest, and that Trump will expose it.)

Which leaves the man not on the ballot in the first four contests, but spending enough money to force himself into contention: the former New York mayor Michael Bloomberg. A plutocrat former Republican with a troubling record on racial bias in policing should be a no-hoper for the Democratic nomination. But the assumption that the mega-billionaire would get under Trump’s skin like no other candidate has made plenty of Democrats take a look all the same.

Such is the nature of this race, in which Democrats place less weight on their own preferences than on what they imagine are the preferences of others – a process of second-guessing that left many in New Hampshire frozen with indecision until the very last moment. Democrats feel a great weight of responsibility in making this choice. They know they have only one chance left to stop Donald Trump – and they all know what’s at stake.

• Jonathan Freedland is a Guardian columnist