Supporters of Italy’s centre-left Democratic party (PD) breathed a sigh of relief on Sunday evening as their candidate in Emilia-Romagna saw-off competition from Matteo Salvini’s far-right League to win the regional elections. That defeat here was even a prospect, though, shows just how much Italy’s political geography has changed.

Historically Emilia-Romagna, in the north of the country, is a bedrock of communism and has had an unbroken string of leftist governments since the second world war. But the left has been losing ground since the financial crisis of 2008 – as in so much of the continent – with austerity-stricken rural communities and provincial towns in particular drifting rightwards. Winning in the region would have been a real coup for Salvini, who took personal control of the local campaign.

His plans may have been thwarted, but there is little to suggest that Salvini is losing momentum. While the League may have failed to win over the region as a whole, it has consolidated support in some of Emilia’s major cities. Similarly, at a national level the polls still suggest Salvini’s party is well-positioned to form a majority government in coalition with other rightwing parties, including the far-right Brothers of Italy and Silvio Berlusconi’s Forza Italia when the opportunity arises. Such a scenario was reinforced by the results of another vote on Sunday in the poor southern region of Calabria: while the League won just 12%, it also oversaw collaboration among the other rightwing parties to snatch the territory from the incumbent PD governor.

If the left has been able to weather the storm at all, this is despite rather than because of the PD’s own campaigning. Instead, it was up to an autonomous social movement called “the Sardines” to secure the result. Formed last November by a group of twentysomethings in Emilia-Romagna’s capital Bologna, the independent initiative called on citizens to congregate in their local piazzas with homemade placards of the eponymous fish, to symbolise solidarity, pacifism and opposition to divisive and violent politics. It might sound gimmicky, but the Sardines struck a chord. In just three months, the movement succeeded in organising demonstrations in 90 cities, the largest mobilisation of civil society in the history of the Italian republic. Their efforts were vindicated. In 2014 turnout in Emilia-Romagna’s regional election was 37.7%. This year it jumped to 67.7%. This spike is almost certainly down to the Sardines, without whom the PD may well have lost control.

The most immediate implication of Sunday’s result will be to prop-up the current government, which now looks set to endure for the foreseeable future. Again, this has less to do with the virtues of the PD itself than the comparatively dire straits of their lead coalition partners, the Five Star Movement (M5S) – which was happily partnered with the League before Salvini broke ranks in a botched attempt to grab power for his own party last August.

In the 2018 national elections, the self-styled anti-establishment populists M5S won 33% of the vote, making it the country’s largest single political force. Since that peak, however, its support has been in freefall. On Sunday the M5S won 7.4% of the vote in Calabria, and a meagre 3.5% in Emilia-Romagna. Its attempt to court both left- and rightwing voters, and failure to deliver on key policy pledges, has seen a steady haemorrhage of its largely liberal base. Even before the weekend’s vote the party’s leader, Luigi Di Maio, stepped down from his position, recognising the need for the movement to adopt a more consistent ideological stance – this is most likely to mean a move to the left.

With the M5S in chaos, and left-leaning voters swinging to PD, Sunday’s elections have confirmed a shift in the power balance inside the coalition government. Having picked up leftwing swing voters from its senior partners, the PD, the junior partners, will now dictate the political agenda. In a potential sign of the changes ahead, government ministers have already suggested abolishing the “security decree”, a piece of legislation passed by the League-M5S coalition to criminalise NGOs and individuals who aid refugees. Overturning this law is one of very few explicit policy proposals supported by participants in the Sardines movement – the PD is trying to send a message to the congregations in the piazzas that the party is taking their concerns seriously. Meanwhile, the M5S remains the only Italian party that consistently emphasises the climate emergency, and is still working towards some kind of green new deal for Italy. Committing to this in more concrete terms now seems its best chance at avoiding a further collapse in support.

What’s most interesting about these elections, then, is the extent of the Sardines’ sudden influence. By manipulating the main parties’ fears of Salvini this makeshift movement has not only blocked the right: it has effectively made the ruling coalition dependent on their endorsement. For now, the organisers are playing down the possibility of forming their own party. This seems wise. While being outsiders has its drawbacks, it has enabled them to avoid self-defeating echo chambers and foster a remarkable pluralism that has been key to their success.

The Sardines are not here to save the old left. Instead their task is more foundational: to rebuild a culture of political participation, and demonstrate to Italy’s sceptical population that grassroots politics and activism can yield results – especially so soon after the M5S’s own failed experiment in “direct democracy”. That they’ve succeeded in this to some degree, in a country where pessimism has become so deeply ingrained is already a minor miracle. If this belief in collective action can be maintained, the League’s road to power may not be as smooth as the pollsters would have it.

• Jamie Mackay is a writer and translator based in Florence