Who are the incumbents?

Despite taking the greatest share of votes in April 2019's general election, Spain's center-left Socialists (PSOE) have been unable to secure the numbers to form a majority government. 

Now, after months of failed negotiations with rival parties, Spaniards will return to the polls for the fourth time in as many years after acting Prime Minister Pedro Sánchez called new elections in September in a bid to break the country's political impasse. 

The Socialists' leader Sánchez, 47, has been in the role of caretaker prime minister since his party called a vote of no confidence in his predecessor Mariano Rajoy, ousting his right-wing Popular Party (PP) from government in June 2018 after several members were implicated in an ongoing nationwide corruption scandal.

Sanchez's tenure has been marked by the fallout of 2017's Catalan independence vote and his party's inability to form a coalition. With none of Spain's five major parties expected to gain the necessary 176 seats for a majority in Sunday's election, the future of Spanish politics is still very much up in the air. 

What are they known for?

Since last year's elections, the Socialists have attempted to appeal to their traditional center-left voter base by increasing the number of women in cabinet positions and raising Spain's minimum wage by 22 percent, the largest pay rise for low-income earners in 40 years.

However, Sanchez's inability to negotiate an alliance with Unidas Podemos has left the Socialists unable to implement their desired structural reforms around labor regulations, education, the country's state broadcaster RTVE, and euthanasia.

On the most significant issue of this weekend's election, the Catalan independence question, Sanchez has pledged to de-escalate tensions in the north-eastern autonomous region following large-scale protests in response to the jailing of nine Catalan separatist leaders in October. 

However, he has been criticized by rival parties and Catalans for an alleged lack of support for citizens injured in the demonstrations and the decision not to meet with Quim Torra, president of the Catalan government, while on a visit to the region last month.

While Sanchez has formerly recognized Catalonia and the Basque Country as nations within Spain, he has pledged to penalize the organisation of what he has called illegal independence referendums.

Who are the opposition?

The Socialists' traditional rival, the conservative Popular Party (PP), is led by Pablo Casado, the party's former communications chief.

Elected as leader in July 2018 following his party's embroilment in the Gurtel corruption case, Casado's rise to power has been seen as a move to the right for PP, with the 38-year old calling for reform of EU freedom of movement legislation and pledging to cut corporation and income tax on a national level. He has taken a firm position against the Catalan independentists.

Coming just behind PP in Spain's last election, Albert Rivera's liberal Ciudadanos (or Citizens) party has also proposed a more hardline approach to the situation in Catalonia, stating the region should have its autonomous status removed and its government president sacked. 

Rivera, a free-trade advocate and social media enthusiast, has marketed himself – particularly through his millennial-friendly Twitter account – as Spain's centrist option, with his campaign rallying cry, "España en marcha" (Spain on the Move), a clear reference to French President Emmanuel Macron's En Marche (Forward) party. 

Standing in stark contrast to Rivera's messaging on Catalan independence and the economy is Unidas Podemos' Pablo Iglesias, who is running on an anti-austerity, socially liberal ticket. 

Sympathetic to Catalan's separatist movement, Unidas Podemos was close to forming a government with Sanchez's party following April's elections, with Iglesias even agreeing to the Socialists' stipulation that the 41-year-old academic would not take a place in the cabinet if it meant securing a coalition. 

Spain's leftist vote could be split by the emergence of Más País (More Country), led by former Podemos policy secretary Inigo Errejon. 

On the other side of the political spectrum lies the populist, anti-immigration party Vox, headed by former PPer Santiago Abascal. 

Despite rejecting the far-right moniker, Vox's anti-Islam rhetoric and anti-immigration policies – including pledging to deport legally settled migrants from Spain if they break the law – echos the politics and growth of other far-right parties across Europe. 

How has the economy been doing?

Despite a five-year slump following the 2008 economic crash, Europe's fourth largest economy has fared comparatively well since 2013 in comparison with many of its European partners. 

In a document submitted to the European Commission in October, Sanchez's party said it expects economic output to grow by 2.1 percent this year, down from previous forecasts of 2.2 percent, with growth forecast at 1.8 percent in 2020, down from 1.9 percent.

But despite the trim in forecasting, this puts Spanish growth far above the projected 1.1 percent growth rate for the eurozone. 

However, unrest in Catalonia, the country's largest economic region, and four years of minority governments have caused growing financial uncertainty, with October's protests hitting both Barcelona's retail and tourism industries.

Investors are hoping for a strong government to emerge from Sunday's elections in the hope a new budget will stabilize the Spanish economy. 

What are the other issues in the campaign?

As is to be expected, Catalan independence has been the key issue, with four of the major parties – the notable exception being Unidas Podemos – incorporating the word "Spain" into their campaign slogans, pointing heavily to the significance of national integrity in this weekend's vote. 

And, while PP and Ciaduados have capitalized on anti-separatist sentiment around the country after October's explosive protests, Vox has taken Spanish nationalism one step further, focusing the conversation on what they have called a "progressive dictatorship" around the topics of  immigration, patriotism, and identity politics. 

Beyond the question of national identity, joblessness remains an important issue to the Spanish electorate.

While unemployment has dropped steeply since the historic highs of nearly 27 percent following the 2008 financial crash, rival party leaders recently berated acting prime minister Sanchez during a televised debate for presiding over the biggest rise in unemployment in Spain since 2012. 

While the Socialists are still predicted to take the lion's share of the vote this weekend, last month's joblessness statistics were seen as one of the reasons for their dip in the polls. 

However, with all forecasts still pointing to an inconclusive result on Sunday, it is likely Spain's politicians will have to return to the coalition negotiation table and try again to strike a deal. 

Otherwise, Spaniards could be heading back to the ballot box much sooner than they would like.