The far-right Vox party took 12 seats in the Andalucían regional election on Sunday, becoming the first such group to triumph at the ballot box since Spain’s return to democracy after the death of Francisco Franco in 1975.

The small but increasingly vocal party, which opposes Catalan independence and has vowed to crack down on immigration and abortion, exceeded all predictions and could now hold the key to the formation of the next government of the populous southern region.

While the Spanish Socialist Workers’ Party (PSOE) won the election, taking 33 of the 109 seats in the regional parliament, its support collapsed in the heartland it has ruled since 1982.

Even with the support of the Podemos-led Adelante Andalucía coalition, which won 17 seats, the PSOE would still be short of the 55 seats needed for a majority in the regional parliament.

The conservative People’s party (PP) took 26 seats on Sunday, while the centre-right Ciudadanos party won 21. Were the two rightwing parties to join forces with Vox, they would jointly command a majority, with 59 seats.

While Vox’s breakthrough marks a turning point in Spanish politics, it is symptomatic of the wider fragmentation of left and right. Before the emergence of the Podemos and Ciudadanos parties, voters had been faced with an electoral duopoly. If you were leftwing, you voted for the PSOE and if you leaned to the right, you voted for the PP.

Although the PP was founded by Francoist ministers, it had, until recently, managed to offer a home to the entire Spanish right, from centrists to extreme rightwingers still hankering after the dictatorship.

However, just as Podemos has given PSOE voters an alternative, so have Ciudadanos – and now Vox – provided PP voters with a different option.

By the time the corruption-mired PP government of Mariano Rajoy was forced from office over the summer, Ciudadanos had already begun to swing further to the right.

The party’s tough stance during the Catalan independence crisis paid off and Ciudadanos finished first in last December’s snap regional election. The PP, meanwhile, saw its seat count dwindle from 11 to just four.

Events in Catalonia have also served Vox, which wants to ban Catalan separatist parties and suspend the region’s self-government “until those who tried to carry out a coup are defeated”.

The party has also been campaigning hard on immigration in a year that has seen a record number of migrants and refugees arriving on Spain’s southern shores.

It wants all illegal immigrants to be deported and is calling for the repatriation of any immigrant who commits a crime in Spain.

Vox’s hardline position has proved contagious, with both the PP and Ciudadanos vowing to protect Spain’s borders in the hope of ensuring their voters don’t defect to Vox.

Pablo Casado, who replaced Rajoy as PP leader in July, upped the nostalgic rhetoric during the Andalucían campaign, looking back fondly on Spain’s imperial past and claiming: “We didn’t colonise. What we did was achieve a greater Spain.”

The collective rightward lurch has put paid to the old notion that Spain’s recent memories of Francoism had inoculated it against the far right.

Recent events – especially the tensions over Catalonia – have given rise to a nationalist narrative in a country where national identity has traditionally been weak and subsumed by regional identity.

“It seems that a lot of people who are leaving the PP for Vox feel that the party hasn’t been tough enough on the Catalan issue,” said Luis Cornago, an analyst at the political risk advisory firm Teneo.

Vox will now be working hard to build on its Andalucían momentum in the run-up to next May’s municipal, regional and European elections.

A campaign video released last month showed the Vox leader, Santiago Abascal, seated on a horse, leading followers across the landscape of southern Spain.

The caption, a reference to the long struggle to end centuries of Moorish occupation, was: “The reconquest will begin on the soil of Andalucía.”

However, as the video makes clear, Vox lacks some of the sophistication of its rightwing international allies.

“If you see a far-right leader in other parts of Europe, they might be an extremist, but they will portray themselves in a much more appealing way,” said Cornago. “Some of these people are good; you may disagree with them, but they know how to do it.”

On Sunday, Marine Le Pen tweeted her congratulations to “our Vox friends, who have achieved a really significant result in Spain for a young and dynamic movement”.