Sánchez wins elections, but needs coalition partners to govern

This still leaves him short of the 176 needed for an outright majority, but the results gained by the main right-wing parties mean he has little risk of being trumped by a coalition agreement between them.

For the first time since Felipe González's reign, which ended in 1998, the socialists have won the general election in 40 of Spain's 52 constituencies, and also acquired the most seats at the Senate, with 123.

In total, the PSOE gained nearly 7.5 million votes in an election which saw a much higher participation than the one in summer 2016 – 75.75% of the electorate, as opposed to 66.48% last time – representing 28.68% of ballots.

The right-wing PP has plunged in popularity – the 123 seats gained in 2015 and the 137 won in 2016 under Mariano Rajoy has now dropped to 66 under Pablo Casado, with just under 4.4 million votes, or 16.7% of ballots.

Until the 2015 elections, voting was always a straight contest between the 'Big Two', the PP and PSOE, but the emergence of powerful independent parties means the likelihood of any one outfit gaining a clear majority is now very remote; in fact, centre-right Ciudadanos is now tailgating the PP with 57 seats, resulting from 4.14m votes, or 15.86% of ballots.

Ciudadanos, who attempted to pact with the PSOE in 2015 – provoking fury among left-wing Podemos who voted down Sánchez's bid to become invested as president – has now stated several times that it has no intention of backing the socialists and that its main aim in the election this time around was to get the PSOE out of office.

It was clear in its intention to pact with the PP if the numbers added up, despite having been very much against them when they were in power from November 2011 until June 2018.

The PP had not ruled out a pact with far-right Vox if the calculations were in their favour – a factor that may have brought about the loss of 71 of the party's seats – and Europe as a whole was starting to show concerns about the possibility of the alt-right gaining a hold in national Parliament for the first time since the death of dictator General Franco.

Vox, which has been on the political circuit for over five years but has never yet run in a general election, earned 12 out of 110 seats in Andalucía's regional Parliament in December, and the southern region, along with Madrid, saw the most votes in Spain going towards the anti-immigration independents.

With one in 10 of Sunday's votes across the country, or nearly 2.7 million, Vox's being a non-established party meant it needed far more ballots per seat than the much larger ones, giving them 24 in total.

This is enough to show European leaders may be right to show concern, but not enough to have any real impact – even joining forces with the PP and Ciudadanos, which had previously appeared less keen to do so, the total number of MPs between them would only come to 147, or 29 short of a majority needed to govern.

Possible coalition agreements

Sánchez has said he will attempt to govern in his 123-seat majority without forming a coalition, fearing being held hostage to other parties' individual demands, but this could leave him in a weak position and it is likely he will have to seek backing elsewhere.

PSOE supporters have already demonstrated in the street, calling for their party of choice to resist any temptation to pact with Ciudadanos – an option that would be a simple solution to a full majority for Sánchez, giving the two together 180 seats.

The most likely path back to the presidency for Sánchez will be joining forces with left-wing Podemos, which has gone from being Spain's third-largest political power to fourth, dropping from 72 seats to 42.

Podemos supported Sánchez in June when the PSOE launched a no-confidence bid against the PP following the latter's corruption charges, ousting Rajoy from the presidential seat and placing the reins in Sánchez's hands, despite his party's only holding 85 seats out of 351, forcing him to appeal to the main Catalunya separatist parties for support.

If Podemos agrees to go into coalition with the PSOE this time, their total of 165 seats will mean persuading another 11 MPs to back Sánchez in the in-house voting and give him back the keys to the Moncloa Palace, the official presidential residency in Spain.

Possible supporters could be Valencia's left-wing regional party Compromís, which has been co-governing the three-province territory since 2015 with the PSOE-PSPV, the region's branch of the socialists.

Compromís has only gained one seat in national Parliament, as has the Cantabria regional party, PRC.

The Canarian Coalition backed Mariano Rajoy's investiture in 2016, but reluctantly, and only because Spain's 315 days without a government had already meant the annual budget could not be approved and the islands needed financial stability – a stance that could mean Sánchez may be able to rely on its two MPs for a vote in favour of his forming a cabinet.

The Basque National Party (PNV), not known for being a supporter of the PSOE's direct rival, the PP, in the past, may also lend the approval of its six MPs.

Only one more favourable vote will be needed, which could come from one or all of the four MPs from the Basque Reunification Party, EH Bildu.

This would mean Sánchez's not having to rely on the Catalunya separatists – Junts per Catalunya (JxCat), a centre-right formation with seven seats in Parliament, or the Catalunya Left Republicans (ERC), with 15, after gaining the highest number of votes in the four-province region.

Whilst these parties enabled Sánchez to gain power in June, they withdrew their support when it came to voting in the PSOE's 2019 budget, forcing the president to call a general election.

And the PP and Ciudadanos have, since then, been accusing Sánchez of pandering to the separatists and claiming a vote for the PSOE would give Catalunya a free rein in its independence bid – despite Sánchez's having insisted he would not allow a legal referendum.

It would be in the PSOE's best interest, therefore, in political terms and in the eyes of Spanish society, to avoid having to rely on JxCat or the ERC, and it looks likely he may be able to achieve this.

Appealing to Compromís, the PNV, the PRC and EH Bildu is almost certain to mean their support comes in exchange for a pledge to provide extra funding to the Basque Country, Comunidad Valenciana and Cantabria – and Valencia has long criticised national governments of either colour for its being sorely under-financed.

Being one of Spain's most densely-populated regions and a key tourism hotspot, along with Andalucía, the Balearic and Canary Islands, Catalunya and Madrid, Valencia has not been receiving enough cash per head to provide the necessary infrastructure and services to its residents or holidaymakers, despite providing a sizeable proportion of the country's income.

Among likely requests if Compromís backs Sánchez is immediate funding and approval for a direct train link along the coast between the cities of Valencia and Alicante, serving both airports, given that this was dismantled in 1974 amid promises of a bigger, better and more efficient transport connection, which never materialised.

The earliest a presidential investiture is likely to take place is June – now, Sánchez will have to commence talks with his likely coalition partners, and King Felipe VI will formally invite him to form a government.