It was only five years ago that Spain’s break-out party Podemos became a dazzling new lodestar for Europe’s lost and troubled left. But with a snap election just weeks away, it now risks a crash as spectacular as its rise. Has the leftwing populist model of ponytailed rebel Pablo Iglesias and his gang of talented young thinkers, so admired by many Jeremy Corbyn backers and others around Europe, proved a failure?

Polling suggests the party is in deep trouble. Podemos once led the polls and very nearly snatched leadership of Spain’s left from prime minister Pedro Sánchez’s Socialist party at the 2016 elections, but it is now only the voters’ fourth favourite party. The meltdown means it is slated to lose half its deputies, while Sánchez gallops ahead, taking well over twice as many votes as Podemos on 28 April.

The far-right populist party Vox is now also snapping at its heels, adding ideological insult to electoral injury. In April, Vox will target the same working-class city neighbourhoods as Podemos – claiming that immigrants and Catalan separatists, not austerity, are the problem. Mainstream parties will undoubtedly argue that this is merely one populism replacing another. That misses the point.

Although Spain has changed since Podemos picked up the banner of the indignado protesters and charged dramatically on to the scene in the 2014 European elections, the damage is largely self-inflicted. In the latest of a series of internal bust-ups, Iglesias’s former No 2, Iñigo Errejón, is leading an alternative coalition into elections for the powerful Madrid regional government in May.

The split matters partly because Errejón was, after Iglesias, the most visible and charismatic of the talented young leaders who created Podemos. Such was the passion they provoked that the two politicians could do stadium tours, filling seats with excited supporters chanting “Yes we can!” Those queuing to see them afterwards were just as keen to plant a kiss on the boyish-looking Errejón as they were to grab a selfie with Iglesias.

More importantly, Errejón was also a key theorist in a party that pledged to break moulds and shed the shackles that had kept the reforming left out of power. He was the most forthright proponent of a philosophy of popular “transversal” coalitions that knitted together a wide variety of groups opposed to the status quo in one of Europe’s most corrupt and unequal societies. This allowed Podemos to channel the rage of the spontaneous indignado protests, which had occupied city squares in 2011. It also prevented it repeating the doomed coalitions routinely put together under the dead hand of Spain’s communist party. Everybody was welcome, the message became, under Podemos’s bright, purple-coloured umbrella.

‘Pablo Iglesias’s former No 2, Iñigo Errejón, is leading an alternative coalition into elections for the powerful Madrid regional government in May.’ Photograph: Juan Medina/Reuters

That message is no longer so clear. Indeed, the party appears to have forgotten that its success has always depended on the fact that it is part of a much broader movement of leftwing regeneration.

Nowhere is this more visible than in Spain’s two greatest cities, Madrid and Barcelona. These are where Podemos holds greatest sway, having helped to form the local coalitions that brought Madrid mayor, Manuela Carmena, and her Barcelona counterpart, Ada Colau, to power. The two cities, home to 10% of the country’s population, are the main laboratories for Podemos-style policies in Spain. They are also, traditionally, a launch-pad for political change.

Both mayors have performed remarkably well as they seek to make cities more liveable, rather than merely richer. Carmena has even pulled off the apparently impossible trick of reducing the debt inherited from big-spending rightwing mayors without instituting austerity. Charges that the new left is radical, dangerous and irresponsible now ring hollow.

Neither mayor allows herself to be bossed by Podemos, a party that is only half-joking when it repeatedly references the power battles waged in Game of Thrones. Colau remains on friendly terms, but the relationship with Carmena has soured as Podemos has shed allies.

Errejón has joined Carmena and her alternative umbrella group, Más Madrid. This will compete with Podemos (and the communists who are now its most important coalition allies) for votes in the regional elections in May. To confuse things further, Errejón still claims to be a loyal Podemos member.

Monica Oltra, the deputy premier of Valencia’s regional government, has already said that her Compromís party, a key local ally, will not repeat an electoral coalition with Podemos in the April general election. En Marea, a similar ally in Galicia, has also walked away. As a result, Podemos’s broad coalition looks increasingly skinny and self-centred.

Podemos should also ask itself why the three most powerful women on Spain’s new left – the two mayors and Oltra – all operate outside the party. Even within Podemos, and despite gender-balanced quota systems, women complain that the party suffers from an overload of testosterone.

The only really visible woman in Podemos is now Irene Montero, the party’s spokeswoman in parliament. She and Iglesias are a couple. A recent decision to raise their children in a generously-sized country house with a swimming pool has helped take the shine off a party that claims to represent ordinary Spaniards. Podemos members, when consulted, voted to keep them in their positions. Ordinary voters, unused to such luxury, will not be so understanding.

The Podemos story remains remarkable and the party has a history of proving naysayers wrong. A year after its foundation, it packed Madrid’s Puerta del Sol square with supporters as Iglesias told the crowd to challenge the monsters of “financial totalitarianism” and “take their dreams seriously”.

That was four years ago, when the damage caused by the global financial crash and a burst construction bubble at home was rawer than it is now. In December, a Podemos-led coalition lost a third of its votes in regional elections in southern Andalucia. Instead of asking Podemos to take over the traditionally leftwing regional government, voters in one of Spain’s poorest regions ushered in a rightwing government backed by Vox. When that happens, something has gone badly wrong with the “Podemos revolution”.

• Giles Tremlett is a journalist and author based in Madrid