Voters in Spain go to the polls Sunday in an election that could make the country the latest European Union member to swing to the populist right, a shift that would represent a major upheaval after five years under a left-wing government.

Here's what you need to know about the vote.
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WHAT IS AT STAKE?

Opinion polls indicate the political right has the edge going into the election, and that raises the possibility a neo-fascist party will be part of Spain's next government.

The extreme right has not been in power in Spain since the transition to democracy following the death of former dictator Francisco Franco in 1975.

With no party expected to win an absolute majority, the choice for voters is basically between another leftist governing coalition or one between the right and the far right.

The right-of-centre Popular Party, the front-runner in the polls, and the extreme right Vox party are on one side.

They portray the vote as a chance to end “Sanchismo” — a term the PP uses to sum up what it contends are the dictatorial ways of Socialist Prime Minister Pedro Sánchez, the left's radical ideology and numerous lies by the government.

In the other corner are the Socialists and a new movement called Sumar that brings together 15 small leftist parties for the first time. They warn that putting the right in power will threaten Spain's post-Franco changes.
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WHY WERE EARLY ELECTIONS CALLED?

Socialist Prime Minister Pedro Sánchez called the early election a day after his Spanish Socialist Workers' Party and its small far-left coalition partner, Unidas Podemos (United We Can), took a hammering in local and regional elections May 28.

Prior to that, Sánchez had insisted he would ride out his four-year term, indicating that an election would be held in December. But after the May defeat, he said it was only fair for Spaniards to decide the country's political future without delay.
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WHAT HAPPENED SINCE MAY 28?

The Popular Party emerged from the local and regional elections as the most-voted party by far, giving it the right to take office in all but a handful of towns and one or two regions.

Since then, the PP and Vox have agreed to govern together in some 140 cities and towns as well as to add two more regions to the one where they already co-governed.

The Socialists and other leftist parties lost political clout across the country, but after weathering the initial shock, they have regrouped and recovered some ground, leaving the vote outcome Sunday still an unknown.
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WHAT DOES IT MEAN FOR EUROPE?

A PP-Vox government would mean another EU member has moved firmly to the right, a trend seen recently in Sweden, Finland and Italy.

Countries such as Germany and France are concerned by what such a shift would portend for EU immigration and climate policies.

Spain's two main leftist parties are pro-EU participation. On the right, the PP is also in favor of the EU, but Vox is not.

The election comes as Spain holds the EU's rotating presidency Sánchez had hoped to use the six-month term to showcase the advances his government had made. An election defeat for Sánchez could see the PP taking over the EU presidency reins.
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WHAT ARE THE CAMPAIGN THEMES?

The campaign has been dominated by mudslinging from all sides, with both the left and right accusing each other of lying about their policies and past records.

The PP has managed to put Sánchez's honourability in question by highlighting the many U-turns he has made and his alliances with small regional secessionist parties, something that alienates even some left-wing voters.

The left has sought to convince voters that there is little difference between the two right-wing parties and that a victory for them would set Spain back decades in terms of social progress.

Nearly every poll has put the PP firmly ahead of the Socialists and Vox ahead of Sumar for third place.

But 30 per cent of the electorate is said to be undecided.

With the election taking place at the height of summer, millions of citizens are likely to be vacationing away from their regular polling places.

But postal voting requests have soared, and officials are estimating a 70 per cent election turnout.
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IS THERE ANY CHANCE FOR A SURPRISE?

A surprise factor that could upset poll predictions is Sumar: the brand new, broad-based movement of 15 small left-wing parties, including Podemos and prominent social figures.

Sumar is headed by highly popular Labour Minister Yolanda Díaz, who is also the second deputy vice president and the only woman among the leaders of the four main parties.

This is the first time small left parties have ever come together on a joint ticket in Spain.

Their earlier fragmentation was blamed for many of the town and regional losses in the May election, and they hope that joined together they can make a bigger showing.

Sumar's big goal is to beat out Vox for the potential king-making third place finish. That would allow Sumar to give valuable support for another leftist coalition government.

Surveys consistently suggested during the campaign that an absolute majority for Popular Party and Vox is very possible.