Spain heads to the polls Sunday for its fourth election in as many years, in a vote overshadowed by the ongoing Catalan crisis which has boosted support for the far-right Vox.

The November 10 ballot seeks to draw a line under months of political deadlock following an inconclusive April election won by Prime Minister Pedro Sanchez's Socialists but without a majority and unable to form a new government.

And surveys suggest a similar outcome on Sunday, with Sanchez's Socialists seen in pole position but still unable to secure an absolute majority.

The Catalan crisis has changed the game for candidates running in Spain's November 10 election
The Catalan crisis has changed the game for candidates running in Spain's November 10 election AFP / PIERRE-PHILIPPE MARCOU

This time, however, the emerging political constellation is likely to be less favourable for Sanchez, largely due to the Catalan crisis which erupted in mid-October when Spain's top court jailed nine separatist leaders over a failed 2017 independence bid, sparking a wave of violent protest.

Polls show the unrest has played into the hands of Vox, which had been a very marginal player until April when it entered parliament with 24 seats in a remarkable revival for the far-right which had spent decades in the doldrums since Francisco Franco's death in 1975.

In recent days, Sanchez has repeatedly raised the alarm about Vox's "aggressive ultra-rightwing" policies, warning the party would drag the country back to the dark days of Franco's dictatorship.

The Catalan separatist crisis has overshadowed the election
The Catalan separatist crisis has overshadowed the election AFP / Pau Barrena

"It is crucial that we mobilise for this election to stop the far-right," he said.

Images of Barcelona in flames with masked youths fighting running battles with riot police have played firmly into the hands of Vox, which wants to ban all separatist movements and has astutely emphasised the national unity card.

"Catalonia has been the main issue and this seems to have benefited Vox because it has taken a very tough approach which has attracted the most hardline rightwing voters," said Teneo analyst Antonio Barroso.

Polls suggest the faction could double its showing and take nearly 50 of the parliament's 350 seats.

As the nightly protests continued, Sanchez came under increasing pressure from the rightwing conservative People's Party (PP) and the centre-right Ciudadanos to suspend Catalonia's autonomy and remove its separatist president Quim Torra.

Key dates in Spanish politics since 2015.
Key dates in Spanish politics since 2015. AFP / Robin BJALON

Despite exercising restraint, Sanchez has toughened his rhetoric and has sent reinforcements to the region to prevent protesters from picketing polling stations.

Although all rallies have been banned on the eve of the vote, protest group Democratic Tsunami -- which swamped Barcelona airport with 10,000 protesters -- has called for Saturday to be a day of civil disobedience across the region.

There is a risk Sunday's vote may end up prolonging the political paralysis that has gripped the eurozone's fourth-largest economy since the December 2015 election which saw Ciudadanos and the far-left Podemos entering parliament, ending three decades of bipartisan hegemony by PP and the Socialists.

Even by joining forces, neither the left -- the Socialists, Podemos and newcomer Mas Pais -- nor the right -- PP, Ciudadanos and Vox -- are expected to win enough seats to secure a majority, polls predict.

Last time, the Socialists and Podemos spent months locked in talks but were unable to bridge their differences, sparking bitter recriminations that would be tough to overcome.

To be sworn in as premier, Sanchez would need the support of 176 lawmakers, a good 50 more than his Socialists are predicted to win in Sunday's ballot.

That would leave only one other apparent option: for the PP to abstain in any investiture vote, allowing Sanchez to form a minority government with outside support from Podemos.

The fear of yet more elections will "force the parties at the last minute to negotiate an abstention", said Jose Ignacio Torreblanca, head of the Madrid office of the European Council on Foreign Relations think-tank.

But Barroso said such a minority government "would never complete its (four-year) term".

"And with such instability, no reforms could be undertaken that would prepare us for the next recession," he warned, referring to the slowdown of Spain's economy and its recent negative employment figures.