CAN you “feel the Bern”? Hillary Clinton certainly can.

The first votes in the race to the White House were cast a week ago in Iowa. From now until July’s delegate conventions, voters in every US state will get the chance to help choose who goes head to head in November’s presidential election.

Both major parties have messy battles ahead. The possibility of far-right reality TV host Donald Trump clinching the Republican nomination is terrifying millions of people appalled by his populist racist and sexist hate-speech.

Like Boris Johnson, the more Trump plays the clown, the more people love him. He also shares the London mayor’s “Heineken effect,” reaching parts of the electorate other rightwingers can’t — not to mention both of their odd hairstyles.

But people around the world breathed a collective sigh of relief when Texas senator Ted Cruz pulled off a stunning victory on Monday despite trailing in the polls, forcing “The Donald” into second place.

It was a classic case of the “ground war” trumping Trump’s “air war.” Arch-conservative Cruz — think a less aristocratic but just as skin-crawling George Osborne — is an experienced political operator who knows that all the money and media exposure in the world means nothing without troops on the ground handing out fliers, knocking on doors and dragging voters to the booth.

But it’s in the Democratic Party that the real struggle for the country’s soul is being fought. Clinton has long been the favourite, and is desperate to avoid a repeat of 2008, which saw the nomination slip from her grasp after a successful grassroots insurgency to her left led by the relatively unknown senator Barack Obama.

The turning point in that contest was Iowa, where Obama picked up the most votes. Suddenly he was seen as a credible candidate, and Clinton’s air of invulnerability was punctured.

It’s worth remembering that, before Iowa, even the black community in the US didn’t believe Obama could win, and their support switched from Clinton almost overnight.

Eight years ago, a lot of people didn’t think that the US was ready for a black president. Obama was “unelectable,” they said.

Bernie Sanders, senator from Vermont, faces the same criticism today. The US will never vote for a self-declared socialist, insist the pundits.

But his virtual tie with Clinton in Iowa proved them wrong. The result was so close that several districts decided delegates by flipping coins — as is the Iowa way. Latest polls put them neck and neck nationally.

Bernie is widely predicted to win tomorrow’s clash in New Hampshire, which would blow this two-horse race right open. And new numbers show him performing better against every Republican candidate than Hillary, who now seems like a cross between Yvette Cooper, Cherie Blair and Theresa May.

Cruz’s early win also benefits Sanders, because Democratic voters fearful of a triumphant Trump might opt for what they mistakenly see as a safer pair of hands — although it would be delicious to watch Hillary eviscerate that chauvinist pig in the debates.

But Bernie could quite clearly crush Cruz in a straight left-right fight, backed up by a highly organised mass movement inspired by his anti-inequality, pro-peace vision of a fairer world.

Currently he has almost half a million supporters using his custom-made canvassing app, Field The Bern, which turns smartphones into always-up-to-date voter-ID boards. Countless other volunteers turn up at his phone banks and pound the streets to spread his message.

Young first-time voters especially have been caught up in the #FeelTheBern frenzy, flooding the internet with hard-hitting and humorous “memes” and videos. Bernie basically has the ground war covered.

This may be the most striking similarity to last summer’s #JezWeCan campaign on this side of the Atlantic, but it’s not the only one. Jeremy Corbyn’s opponents insisted he was “unelectable” too — in fact they still do.

You’d think a landslide leadership victory which saw more people give their first preference to Jez than his three rivals combined, not to mention all the Momentum volunteers helping to boost Labour’s vote in Oldham, would put paid to such criticism, but no.

And because they believe that the public will never warm to Labour under JC’s leadership, MPs more worried about keeping their seats than forming a government in 2020 tear chunks out of him publicly day and night — a great example of self-fulfilling prophesy.

But his enduring popularity among party members proves that left-wing voters really are fed up with endless spin, triangulation and pandering to the powerful, and they’re sticking with someone authentic who shows a bit of imagination.

This is encouraging for Sanders, who likewise is showing us that the time is right for a socialist mass movement — even in the US, where “socialism” is almost as scary as “terrorism.” And where the US goes, Britain follows.

Above all, the lesson from Obama — and other world leaders, from Hugo Chavez to Alexis Tsipras — is that with a mass movement on the march you’re only unelectable until people realise you’re inevitable.

• Chat to Charley on Twitter: @charleyallan.