A worker carries a ladder after sticking a campaign poster depicting Spanish Prime Minister and presidential candidate for the Spanish Socialist Party (PSOE) Pedro Sanchez on a billboard close to the village of Viella, Asturias region, on April 12, 2019.

Spanish opinion polls are signalling that the Socialist party could win a large share of the vote in a snap election on April 28 – but not enough for it to govern alone.

Prime Minister Pedro Sanchez called the snap vote in February after Catalan independence parties withdrew their support for the government's budget bill. The move could have worked in Sanchez' favor, however, with opinion polls suggesting that his Spanish Socialist and Workers' Party (the PSOE) will gain far more parliamentary seats.

There are four major parties spanning the political landscape in Spain – the left-wing, ruling Socialists, the center-right People's Party (PP), far-left Unidos Podemos party and the liberal, centrist Ciudadanos (Citizens' Party). There is also now an upstart, populist newcomer in the mix – the far-right VOX party.

According to the latest opinion poll, the PSOE leads by a wide margin and is seen with 31.1% of the votes. Conducted between April 9-11 by social research firm GAD3 for La Vanguardia newspaper, the poll showed the PP trailed with 20.1% of the vote, Ciudadanos was seen with 14.4% and Unidos Podemos with 11.4%. Far-right Vox is seen closely behind with 11.2% of the vote.

A host of smaller parties, including Catalan independence parties, are seen with a smaller percentage of the vote while a large number of voters (26%) remain undecided as to who to vote for, and this could have a large impact on the final result.

As it stands, however, the Socialist Party is expect to win the highest number of seats (around 137-139 seats). But with 350 seats up for grabs in Congress, no one party will gain an absolute majority (of 176 seats) to govern alone so a coalition government is highly likely. That will involve horse-trading between the main parties and smaller partners.

The PSOE could have to rely on Podemos and secessionists in Spain, again, while a right-leaning coalition could be formed by PP, Ciudadanos and Vox. Whatever the outcome, the political scene is seen as highly fragmented.

A key date for voters could be April 23 when a debate between the main party leaders – Pedro Sánchez (PSOE), Pablo Casado (PP), Pablo Iglesias (Podemos), Albert Rivera (Citizens) and Santiago Abascal (Vox) – is televised.