Sanchez, who has reinforced his position after seizing power and ousting conservative prime minister Mariano Rajoy in a no-confidence vote last June now has to forge alliances to govern.

With 123 seats, Pedro Sanchez’s socialists fall well short of the magic 176 needed to form a majority in the 350-seat lower chamber of Spain’s parliament.

This is the division of seats  in the 350 seat parliament with 99.99 percent of the votes counted. 

The socialists have vowed to rule with minority government, relying on the intermittent of political partners but in order for Sanchez to be sworn in as Prime Minister he needs to secure a majority in a parliamentary vote – the investiture vote.

As the leader of the party which won the biggest share of the vote, Sanchez will be invited by King Felipe VI to have the first chance at forming a government. 

Parliament will be reconvened on May 21st when Sanchez’s leadership will be put to the vote. 

If in a first vote Sanchez must win an absolute majority – with at least 176 votes in his favour – to become prime minister but if he fails to do so, then he must only secure a ‘simple majority’ – with more voting for him than against him – to take the role.

Will he forge an alliance of the left, risking precarious support from regional parties including the Catalan separatists, as he has done over the past ten months?

Or will he turn his back on Podemos and instead consider a moderate centrist coalition with Ciudadanos?

Let’s take a look at his options:

Left wing alliance:

The PSOE and Unidos Podemos (UP) are natural bed-fellows and pony-tailed leader of the ‘radical left’ Pablo Iglesias has already pledged to support Pedro Sanchez in his efforts to make a government.

But even with Podemos’s 42 seats, the alliance falls 11 seats short of reaching a majority.

That means that they will have to garner the support of smaller regional parties.

With or without the Catalans?

The Basque PNV will likely support Sanchez with their six seats, add the two seats from the Canaries Coalition (CC-PNC) plus the Cantabria Regional Party (PRC) and Valencia’s Compromis which have one each, and the number in support of Sanchez reaches 175, just one short of a majority vote.  While this isn’t enough to secure his position as PM in the first round he would just need one abstention in the second round vote to become PM.   

But Sanchez could easily form a government with the support of Catalan regional parties. Catalan Republic Left (ERC) win15 seats and Junts Per Catalunya (Together for Catalonia) have 7 seats. 

Together with the other regional parties plus the separatist Basque party EH Bildu that would bring the numbers to above 200 seats.

Remember, Sanchez was able to oust Rajoy and seize power with a no-confidence vote because he had the backing of regional parties including the troublesome Catalan independistas.

But his position was precarious and ultimately he was forced to call early elections after the Catalan parties, angered at the jailing and subsequent trial of separatist leaders refused to support his budget.

So Sanchez will be keen to form a government not reliant on the votes of the Catalan separatists, so as not to be held hostage to their cause.

Alliance of the centre

With Ciudadanos taking the third largest share of the votes, wouldn’t they be best placed to form an alliance with the socialists?

 In an editorial on Monday, daily newspaper El Mundo urged Sanchez to “reach out to Rivera and consider forming a moderate government — which would undoubtedly go down well in Europe — to ensure the stability” of the country.

The Cs 57 seats added to the Socialists 123 seats makes a total of 180 – four over the absolute majority needed and would cut out the need for striking a deal with the regional parties. But it doesn’t look likely.

Ciudadanos leader Albert Rivera, built his campaign on disparaging Sanchez, criticising his attempts to negotiate with Catalan separatist parties in a bid to ease a secession crisis in the northeastern region.

Although nothing has yet been officially stated from Socialist HQ, Sanchez appeared to dismiss the possibility during post-result celebrations on Sunday night.

Met by a crowd chanting “Not with Rivera! Not with Rivera!” outside PSOE headquarters in Madrid’s Calle Ferraz, a smiling Sánchez replied, “I think that’s pretty clear.”

Senior Ciudanos leader Ines Arrimadas told radio Onda Cero on Monday her party could not ally with Sanchez, calling him a “public danger, someone capable of anything” to stay in power.

Poltical deadlock

Everyone is keen to avoid to the months of deadlock that followed the December 2015 election when an inconclusive election saw the PP win the largest share of the vote but without enough support to form a government. Back then, Sanchez first tried and failed to form a government and a second election was called for June, when the PP won enough seats to form a government with the support of Ciudadanos. 

But with regional, local and European Parliament elections scheduled for May 26th, the parties are wary of making any commitments that might put voters off, commentators suggested it was unlikely that a new government be formed before June. 

ANALYSIS: Spain chooses left-wing regional diversity while Vox divides the right