AP Explains: Polarized and undecided, Spain heads to polls An uncertain outcome and the likelihood of a far-right party entering parliament are the backdrop for Spain's most polarized national election in decades

BARCELONA, Spain -- An uncertain outcome and the likelihood of a far-right party entering parliament are the backdrop for Spain's most polarized national election in decades.

A look at why the country's voters were called to cast ballots early and what's at stake when do Sunday:

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HOW DID SPAIN GET HERE?

Socialist Prime Minister Pedro Sánchez came to power last June when he succeeded in ousting the conservative Mariano Rajoy, who retired from politics after he lost a no-confidence vote following a major court ruling in a corruption case that implicated his Popular Party.

Sánchez*s minority government was able to cling to power for a short time. But it failed to pass a national budget in February, resulting in him calling early elections.

Spain*s third national election in four years is the result of the emergence of new parties born from discontentment about the global economic crisis, which has hit Spain particularly hard.

The economy is back growing again, but long gone are the days when the Socialists and Popular Party divvied up most of the electoral spoils.

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WHO'S RUNNING

Sanchez is the front-runner according to all the polling data, but the increased fragmentation of Spanish politics means that the combined forces of two rightwing parties and the upstart extremist party Vox could win the day.

Joining the Popular Party on the right are the pro-business Citizens party, and the nationalist Vox, which defends bull-fighting as an essential Spanish tradition, is hostile to women's rights, and promises to stop illegal immigration.

Voters angry with austerity measures have United We Can on the far left.

Around one-third of potential voters were still undecided heading into the final week of the campaign, according to polls. Up for grabs are the 350 members of the Congress of Deputies, who then choose a government.

Voting stations open at 9 a.m. (0700GMT) and close at 8 p.m. (1800GMT), with results expected a few hours later.

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THE LEFT'S CHANCES

Even if the Socialists get the largest share of votes, their ability to stay in power will likely depend on forging alliances.

Sánchez has hinted that he could end up inviting United We Can to form a center-left government like the one that rules Portugal. There has also been speculation that the Socialists could move the other way toward the center-right Citizens, but Sánchez says that option is "not in his plans."

But it is likely that Sánchez would still need to woo some small regionalist parties into backing him.

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A RIGHTWING COALITION?

Vox is poised to give Spain's Parliament its first openly far-right lawmakers since the 1980s.

Instead of promising to isolate Vox like mainstream parties have done in some other European countries to keep extremists out of government, both the Popular Party and Citizens are radicalizing their messages to stem the flow of voters to Vox.

New Popular Party leader Pablo Casado says the three parties on the right should 3/8pool3/8 their votes if they have enough to kick Sánchez out of the presidential palace in Madrid.

The three right-wing parties have already shown they can join forces.

In January, the Popular Party struck a deal with Vox to get its backing so the Popular Party and Citizens could form a government in southern Andalusia following regional elections.

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WHAT'S AT STAKE?

The incoming government faces the task of dealing with chronic unemployment and dwindling funds for the public pension system.

But the most divisive question is how to deal with the festering political crisis caused by wealthy Catalonia's push for secession.

The Socialists are defending the current model of Spain*s government, which gives its regions sweeping powers to run social services, such as education and health.

But all three right-wing parties — Popular Party, Citizens, and Vox — are vying to tap into the widespread anger in Spain toward Catalonia after its leaders tried to secede two years ago. All three criticize Sanchez for opening talks with the region's pro-secession leaders. Vox has even pledged to recentralize all power from Spain's regions.

Catalan separatists are also aiming to increase their current 17 seats to have more leverage in parliament. Five of the 12 separatist leaders on trial for the failed 2017 breakaway bid are running as candidates from a jail cell.