Final opinion polls, that were published on Monday, tipped the right-wing Popular Party (PP) to win the most seats but without securing a working parliamentary majority.
That could force the PP to form a coalition government with the extreme-right Vox party, in what would be the first time a far-right party holds a share of power in Spain since the end of the dictatorship of General Francisco Franco in 1975.
Al Jazeera’s Sonia Gallego reporting from Madrid said, “Vox is a relative newcomer to the Spanish political scene; nevertheless, they are proving themselves to be disruptors to that traditional two-party system and have even been going into deals with the PP in regional governments as well and councils.
“Vox’s rhetoric around immigration and regional separatism will put it on a collision course with those separatist movements, not just in Basque country but Catalonia as well,” Gallego said.
But the PP’s campaign has stumbled in the final stretch with its leader Alberto Nunez Feijoo facing renewed questions about his ties with notorious drug trafficker Marcial Dorado in the 1990s when he was a senior official in the regional government of Galicia.
Meanwhile, PP and Vox have portrayed the vote as a chance to end “Sanchismo” – a term the PP uses to sum up what it contends are the dictatorial ways of Socialist Prime Minister Pedro Sánchez.
In the other corner are the Socialists and a new movement called Sumar that has brought together 15 small left-wing parties for the first time. They warn that putting the right in power will threaten Spain’s post-Franco changes.
The prime minister has made pacts in exchange for votes in parliament with parties such as EH Bildu, which is linked to the former Basque separatist group ETA, and ERC, which led Catalonia’s 2017 secession bid from Spain.
‘The fight is more ideological’
With that political expediency on display and the demise of centrist parties seen in previous elections, analysts have aid the starker ideological choices have left some voters in a mood to block what they do not like, rather than pick what they do.
Sanchez called the election early after the left took a drubbing in local elections in May, but many voters are also furious at being called to vote at the height of a sweltering summer.
A Reuters straw poll of two dozen voters around the country found many with little enthusiasm for any party and torn between what they viewed as the least bad choice. This sense was reinforced by opinion surveys reporting that about 12.5 percent of people who plan to vote were still undecided on Monday.
Madrid pharmacist Amparo Bonilla, 44, said she would vote for Feijoo – and against Sanchez – to defend democracy, the unity of Spain and “stop terrorists governing us”.
“There is only one motive for voting in these elections – to kick out the psychopath,” she said.
In a bar in the seaside village of Cee in the northwestern region of Galicia a group of men in their 60s told Reuters they would not vote for Vox because it would be “a throwback to the past”, yet all agreed when one man said he did not want EH Bildu either, so he might not trust the PSOE (Socialist Workers’ Party) with his vote.
Spaniards “tend to vote to avoid something, rather than in favour of something,” said Cristina Monge, a political science professor at the University of Zaragoza.
“This time it is more pronounced because the fight is more ideological and the blocs are more defined.”
Leaders made one last pitch for votes during their final rallies on Friday evening.
Feijoo, in a nod to Sanchez’s deals with separatist parties, said a PP win would mean “not being tied down by those who want to separate Spain”.
Sanchez told a cheering crowd “it is possible to win against the right and far-right, and we will do so here in Spain”, while Second Deputy Prime Minister Yolanda Diaz, leader of the far-left Sumar platform, called for votes to ensure Spanish society “does not go back 50 years”.
Polls open at 9am (07:00 GMT) on Sunday and close at 8pm (18:00 GMT), when exit polls will be released. The final result is expected to be decided by less than a million votes, less than 10 seats in 350 parliament, experts have said.