Spaniards head to the polls on Sunday in what is the most unpredictable vote since the country emerged from dictatorship into a fragile, newborn democracy more than 40 years ago.

It may sound like a lazy, simplistic exaggeration. But in this election, the battle is about the very unity of Spain.

The Socialists are set for a decisive victory under current Prime Minister Pedro Sánchez, brought to power only 10 months ago.

All polls show he’ll be well short of a majority. After that all other bets are off.

So Sunday’s election will be more about who will pact with who the day after. The coalition talks that follow will be long and messy.

They’re completely unknown territory for Spain. Spanish news bulletins are buzzing with speculation about what the next government will look like. No-one knows for sure.

The country has never had a governing coalition at national level in modern times.

On the campaign trail, the left and the right have used it as threat, saying their political adversaries in a coalition would put Spain into a state of chaos.

It would mean either a centre-left government between the Socialists and the far-left party Podemos, with its more sympathetic approach to Catalan parties who want to break apart Spain.

Or a centre-right alliance that would swing Spain nearer to the extreme right.

The unpredictable factor comes from the staggering rise of the far-right Vox party. In the 2016 election, they won just 0.2% of the vote. This time round, they could be the kingmakers.

Its policies are straight from the populist playbook. As well as being both anti-immigration and anti-establishment, its main focus is defending national unity, promoting Spanish values and traditions and proudly waving the Spanish flag as a nationalist symbol.

For some, this is a nightmare scenario. For others, the status quo under the current Prime Minister is to be feared more.

“If Sánchez wins, Spain will stop being what it’s been the past four decades”, centre-right Popular Party leader Pablo Casado said in a TV debate this week.

The 38-year-old, photogenic leader of the PP was elected leader last summer, bringing in a more right-wing agenda than his predecessor, Mariano Rajoy.

His party has been forced to move to the right precisely because of the threat of Vox. But it could be a poor strategy. The PP are currently on course to half their number of seats.

Casado’s message to voters is that Spain needs to be rescued from pro-Catalan independence parties that are threatening national unity and holding the country hostage.

Some voters perceive that former leader and Prime Minister Mariano Rajoy wasn’t tough enough on Catalonia.

During Rajoy’s time in office, relations between Spanish and Catalan leaders had been bitter for years. Sánchez has managed to turn down the heat, resuming dialogue and forming a working group.

But in doing so, the opposition has accused him of being both a traitor and danger to Spain.

Sánchez has retaliated, refusing any discussions about independence or a referendum.

Elections had always seemed inevitable.

With a weak government and a far less-than-finished set of reforms, he had no option but to call snap elections back in February.

The straw that broke the camel’s back was Spain’s delayed budget.

It’s the irony of Spanish politics that Mr Sánchez was brought to power by Catalan independence parties – who could be behind his potential, if unlikely, downfall. The same parties then voted down the budget, rejecting conditions for talks between Madrid and Barcelona.

To have got even this far, Sánchez has had a certain amount of luck. He was in the right place at the right time when he became the first Spanish politician in modern times to win a confidence vote, signing agreements with a broad range of parties to bring down Rajoy and his party’s long legacy of corruption.

As Sánchez saw it, his government, with its majority-female cabinet, would be a symbol of a progressive Spain that wants to be the centre of European affairs. With his polished, youthful image, it was straight out of the Macron playbook of politics.

Earlier this week, the four male candidates for Spanish Prime Minister took part in two consecutive nights of late-night TV debates, ending in typical Spanish style well past midnight, before being chewed over even more in post-match analysis.

The elephant in the room all along was Vox.

To coin a phrase, it is on a self-styled conquest to ‘make Spain great again’.

In its view, that means standing against progressive reforms, feminist policies and laws protecting women from domestic violence.

Vox is competing to be the toughest on Catalonia, tapping into anger across Spain after the region’s illegal independence referendum in 2017.

The party supports returning power to Madrid, bringing back echoes of the centrist totalitarianism that Spain lived through under Franco.

Formed back in 2013, it’s taken a while for the party to gain traction. It was catapulted to the national stage only in December, after standing in regional elections in Andalucía.

It won 12 seats and 11% of the vote there, bringing down near 40 years of Socialist rule and becoming the first far-right party in a regional parliament since the Franco era.

In Sunday’s election, Vox is expected to enter Spain’s parliament for the first time with around 30 seats. However, polls could be underplaying the party’s popularity. A ‘hidden’ vote could propel it above fifth place and give them twice as many seats.

Even after this election, Vox will still be a fringe party, but as all populist parties do, they have great influence on their establishment counterparts, which in turn mimic some policy to avoid haemorrhaging votes.

As such, this is already victory for Vox.

Many of its founding members were disaffected Popular Party members.

The party is expected to fare similarly to Podemos, the far-left populist party born at the same time as the protest movement over inequality and Spain’s ailing economy.

Its leader, Pablo Iglesias, is unforgettable. A former university professor, he has a ponytail, rolled-up shirt sleeves and a constantly raised fist in defiance of the political mainstream.

To get a measure of their politics, Podemos endorsed Jeremy Corbyn’s campaign to lead the Labour party in 2015.

Iglesias is looking incredibly likely to support the Sánchez and his PSOE party to keep the centre-left in power. Mr Sánchez sounds warm about the idea of offering them ministries in government and analysts suggest it is the most likely outcome.

Together, they hope they will see off the Popular Party and the signs they will form a centre-right coalition with Vox and Ciudadanos, another party that has been pulled to the right from the centre ground.

Like Vox, Ciudadanos stands firm on Catalonia, where the party was born.

It has its eyes on taking over the Popular Party as the largest centre-right party and has ruled out entering a coalition.

Its leader Albert Rivera, said it was a “national emergency” to get rid of Sánchez.

If Sánchez and the Socialists manage to emerge as the largest party, it will be their first victory in more than a decade.

It will be a show that, against the rules of politics these days, occupying the centre-ground is a vote-winner.

But that won’t be enough.

As we know all too well, polls these days throw up all sorts of surprises.

Vox’s vote could be underestimated, the right could rise, the left could be revived – or the biggest surprise at all, that there is no surprise.

Across Europe, the political establishment is continuing to be rocked. Two-party politics is over and fragmentation rules.

Spain was always the country that most thought would be immune from populist, far-right politics, especially given its still raw fascist past.

That prediction was wrong. Now, Spain is in for a few volatile months.

It’s early days yet for the Prime Minister’s attempt to “break the logjam” in her words by meeting Jeremy Corbyn.

It’s optimistic thinking that the pair can agree on anything. Sit-down talks with the Labour leader could fail as soon as they were announced last night by Theresa May.

Will MPs ever find a majority for any Brexit option, the EU is asking again and again. The odds are slim.

A week today, Theresa May will be standing before 27 watchful European leaders at a summit in Brussels, all hopeful that she has a plan that she thinks enough MPs can finally get behind.

And that, shock horror, could be a softer Brexit.

She’ll be desperate for EU leaders to green-light her request for another extension, moving the no-deal Brexit cliff edge for a second time, from next Friday to 22nd May, by which time she hopes the UK’s withdrawal will be done and dusted – crucially before her red line, the EU elections, which begin the following day.

So in just over a month: agree an extension, get a Brexit deal over the line AND avoid European elections.

It sounds like the stuff of miracles.

And apparently miracles can only be granted by the EU if Theresa May commits to those elections, passing legislation for them for legally unavoidable reasons, but only as a contingency measure if she delivers her deal (explained ably here by my colleague, Mark Stone).

But failure to get Brexit over the line and the UK is looking at European elections and another delay to Brexit.

Quite how long, nobody really knows. The prime minster wants it to be “as short as possible”, but there’s little trust in EU circles in what she says anymore.

Standing beside the Irish Prime Minister Leo Varadkar in Paris, French President Emmanuel Macron said yesterday that a long extension is “far from evident or automatic”.

It would need a clear reason, and a new approach.

So sure enough, on the same day that the hardline leader demanded that Britain tell Europe what it wants and tell them “now”, in the evening the Prime Minister’s podium moment came, heeding the call for some clarity.

Macron’s concern is that the UK could jeopardise the EU’s key institutions on its way out, before a new Commission and its President are elected in the summer. Hence why he reportedly pushed for an even earlier exit of 7th May at last month’s Brussels summit.

The priority must be the workings of the EU and the single market, Macron warned, repeating his now familiar line that the EU can’t be held hostage by Britain’s self-made “political crisis”.

Not every leader is quite so alarmist.

European Council president Donald Tusk tweeted “let us be patient”. This from the same official who called on MEPs in Strasbourg just a week ago to consider a long extension.

All indications are that – through gritted teeth – even Macron would join his EU counterparts in agreeing to a long extension, unanimously as per the voting rules.

Here’s the blunt truth: however much leaders are at pains to point out that there are plenty of other important issues, such as trade and the EU economy, Brexit will drag on for years regardless.

Talks on the UK’s future relationship with the EU will be even more painful and protracted.

Speaking in Paris, Leo Varadkar said that the EU should avoid a “rolling extension”, so the possibility is that it would be a long one that could be shortened, instead of vice-versa.

Another condition – that there is a sliver of a chance that a new approach can get a majority in the House of Commons.

The signals from MPs in Westminster point to a different form of Brexit. One possibility could be remaining in a customs union with the EU – an option that Jeremy Corbyn has been calling for and one which performed best in the series of indicative votes.

Indeed, Macron yesterday cited a customs union as a specific option the European Union would be being open to considering.

It would be a red line for Mrs May, causing a likely mutiny of Cabinet ministers.

But crucially, it’s been the Prime Minister’s stubborn red lines up until now that have, in great part, brought us to this impasse.

The non-legally binding, ambitious document on the UK’s post-Brexit future relationship could be changed “in a period of days and weeks”, according to the EU’s chief Brexit negotiator Michel Barnier, to take into account something like a customs union.

The alternative, however, could be a no-deal Brexit.

No matter how hard the talk is from the French President, he stood next to Mr Varadkar and admitted that their countries – France and Ireland – stand to be the most affected by a no-deal.

“We will never abandon Ireland or the Irish people no matter what happens”, Macron said.

So why would the EU want to be seen to be inflicting so much pain on itself by favouring no-deal over a long extension?

They wouldn’t.

With European elections next month set to be another show of strength for populist, eurosceptic forces across the continent, a no-deal would be like throwing a red rag to a bull.

Italy’s far-right deputy prime minister Matteo Salvini has called Brexit talks “an attempt on the part of Brussels to punish”.

And in the German Bundestag, co-leader of the far-right nationalist party, Alternative für Deutschland, has criticised Chancellor Merkel for being “partly responsible” for Brexit, for her failure to help out the UK.

Talk of no-deal has ramped up across the continent in recent days, amid fears of it happening accidentally.

The EU wants to be seen as being ready for every Brexit eventuality.

It’s why Brussels will begin a series of news conferences later about the effect of a no-deal on everything from food safety to fisheries.

Tomorrow, German Chancellor Angela Merkel will head for Dublin for her own meeting with Leo Varadkar, precisely because she is worried that preparations for a no-deal Brexit in Ireland, with its obvious border complications with the UK post-Brexit, aren’t as advanced as they should be.

There’s lots riding on talks producing a way forward between now and next Wednesday’s Brussels summit.

But as things stand, the UK will leave either with a deal or without a deal in just 9 days.

To really make some headway, Theresa May will finally have to give up her red lines to deliver Brexit.