MADRID (Reuters) - Spain’s Andalusia region kicks off a busy electoral season that could upend national politics, with both the prime minister and his main opponent facing their first electoral tests, and a surging far right poised to win its first seats since the 1970s.

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Far-right success elsewhere in Europe was long unthinkable in Spain, seen as inoculated by the memory of Francisco Franco’s dictatorship, which ended only with his death in 1975.

But polls predict that could change with Sunday’s vote in the southern region, Spain’s most populous and a left-wing stronghold for decades.

The far-right Vox party could win up to five seats in the regional legislature, according to a poll of polls by El Pais newspaper, although there is also a chance it could win none.

“Spain, one of very few European countries with no far-right lawmakers, is catching up,” said Pablo Simon, a political science professor at Madrid’s Carlos III university.

“This could have huge implications on the political system and on the political discourse here,” he said, pointing to the conservative People’s Party’s new leader Pablo Casado’s increasingly tough language on immigration.

Andalusia, which has one the highest unemployment rates in Europe, is the entry point for thousands of Africans who have been reaching Spain by sea in greater numbers as traffic on the other main route across the Mediterranean to Italy has been curbed.

Opinion polls show the Socialists, who have ruled Andalusia since Spain’s return to democracy, winning the most seats yet again, but falling short of a majority with the centre-right Ciudadanos and anti-austerity far-left also vying for seats.

The likely lack of a majority and uncertainty over forming a coalition foreshadows similar expected results in municipal, regional and EU elections in 2019, and a general election which must be held by the following year.

The Andalusia election will be the first electoral test for Socialist Prime Minister Pedro Sanchez, who came to power without an election in June after parliament ousted the conservatives over a corruption scandal.

“Andalusia is the first stop on the route to Socialist victories,” Sanchez said in a rally backing regional candidate Susana Diaz, referring to the series of elections to come.

Speculation is rife in Spain over if, and when, the Socialists, who control fewer than a quarter of parliamentary seats and are still struggling to get the country’s budget adopted in parliament, could call an election.

“A huge victory of the Socialists should be a positive message for Pedro Sanchez and might be considered as an incentive for early elections at a national level,” Natixis analysts wrote in a note.

But senior Socialist party officials brushed off the idea, saying that they were hoping anti-austerity protests this week in Barcelona could convince Catalan nationalists to back the central government’s budget and allow Sanchez to carry on as prime minister.