The ballot will mark the end of the established two-party system that has held sway since the dictatorship of Francisco Franco ended in 1975, ushering in an untested and potentially volatile era of consensus politics.
It will also offer the latest snapshot of the willingness of European electorates to abandon the mainstream centre-right and centre-left, following significant gains by populist parties since October in elections in France and Portugal.
Opinion polls show the governing conservative People’s Party (PP) of prime minister Mariano Rajoy will win tomorrow’s vote but fall well short of an absolute majority.
Rajoy said he would consider a cross-party pact to ensure a stable administration over the scheduled four-year term, but all the main opposition parties have come out against joining the PP in a coalition.
That points to a stalemate that analysts agree would probably disrupt an economic reform that has helped pull Spain out of recession and made inroads into a still stubbornly high unemployment rate.
Many Spaniards view the election as an opportunity to shake up a political establishment they consider inefficient and corrupt.
Cristian Ciudad, who works on a fish stall at Valencia’s fresh food market, said: “The People’s Party and the Socialists have run out of steam. They’ve promised things that they’ve never done and I hope that Podemos will be a change.”
The 22-year-old says most of his friends will also vote for the upstart leftist anti-austerity party because they think it can bring much-needed youth employment.
“If there are no jobs there is no money, and if there’s no money you can’t have a home or any other basic thing,” he said.
But even with one in five voters still undecided, anything other than a PP win would be a major surprise.
The Socialists are expected to come second with Podemos (“We Can”) and a second major newcomer, the liberal Ciudadanos (“Citizens”), vying for third place to become kingmakers in post-election talks.
That prediction makes any of three outcomes possible: either a right-wing or left-wing coalition government or a minority administration.
Socialist leader Pedro Sanchez and Ciudadanos chief Albert Rivera have both rejected the idea of a coalition with the PP.
Rivera said: “It would be a disappointment for millions of Ciudadanos voters if we were to be part of... a government or a pact with parties that represent a way of doing politics from the past.
“It is possible to be in the opposition and back the investiture of another party, but I won’t do it.”
That runs counter to the party’s line hitherto, as it has supported governments from the left or right in five Spanish regions.
A Socialist-led government backed by Podemos is also possible, but the newcomer party’s leader Pablo Iglesias has demanded major concessions to consider that option.
Ciudadanos and Podemos insiders say both parties are looking beyond tomorrow’s vote with their main ambition to keep stealing voters from the PP and the Socialists, giving them little incentive to support a coalition government.