But analysts say Brexit could further empower the anti-establishment, giving those most critical of European unity a boost. That might tip the scale in favor of the radical-leftist party, which had already looked set to oust the Socialists as the main voice on the left.
Spaniards are so angry about the inability of their leaders to work together that experts expect low participation on Sunday and say it is hard to predict the final outcome.
"Participation figures are going to drop with respect to the December election because some are very angry with the politicians and others are confused," said Jose Diez Nicolas, who heads the Universidad Europea's Department of Social Science Research. "It's not that they are undecided. It's that they don't know who to vote for because there are so many messages flying around about who not to vote for."
The Popular Party and the Socialists dominated Spanish governments for decades. But the two upstarts -- the radical-leftist Unidos Podemos and the centrist Ciudadanos -- have shattered the hegemony of two-party politics. Their fresh-faced, 30-something leaders have added an element of energy to a mostly stagnant political landscape.
Pro-business Ciudadanos has so far appealed to socially liberal, urban voters on the right and left.
But Podemos fits into an anti-establishment movement across Europe represented by Brexit, National Front leader Marine LePen in France and Greek Prime Minister Alexis Tsipras — albeit from the radical left. That has made it especially popular with underemployed youth and the working class.
Podemos supported a vote for Britain to stay in the European Union, but analysts say its critical stance on E.U. financial commitments and the status quo would call into question key agreements if it gained decisive power.
The Popular Party gained the most votes in December's election, with 123 of the 350 seats in the lower house of parliament, but lacked a majority. The results reflected widespread discontent with austerity measures after years of recession and accusations of political corruption. The Socialists came in second. Podemos and Ciudadanos placed third and fourth, stripping the two heavyweights of key votes.
Polls suggest a fragmented result in which the Popular Party might win the most votes, with a split between it and Ciudadanos on one side and the Socialists and Podemos on the other -- but again neither with a majority.
The pony-tailed leader of Podemos, Pablo Iglesias, has said he would strike a deal with the Socialist Party to form a left-wing coalition government. But analysts say the Socialists are unlikely to agree to such arrangement, seeing it as political suicide.
Another obstacle would be Podemos's support for a Scotland-style referendum in the northeastern region of Catalonia, which seeks independence from Spain. The three other parties oppose the measure.
But deeper differences separate the two parties.
"There are important discrepancies in economic policies, like the public spending Podemos proposes," said Carlos Barrera, professor of election communication at Universidad de Navarra. "I don't think the Socialists would be willing to sign off on it. Even if along general lines they agree on social issues, the actual application of concrete policies is clearly different."
Meanwhile, acting Prime Minister Mariano Rajoy will look to form his own coalition government with his party's natural ally, Ciudadanos, but could only do so with the approval or abstention of the Socialists. Led by the 61-year-old Rajoy, the Popular Party missed the changing of the guard but still commands confidence, thanks to weathering the economic recession with growth of 3.3 percent in 2015 and 3 percent forecast for this year.
"The only reason the Spanish population forgives [the corruption scandals of] the Popular Party is because they are perceived to be the ones that can carry us through the recovery and ensure that Spain continues creating employment," economist Fernando Fernandez of Instituto de la Empresa explained.
Even so, Iglesias -- and to a lesser degree Ciudadanos' Albert Rivera -- has attracted young voters who a few years ago were calling themselves indignados, or “the outraged,” the origins of the Occupy movement.
"What we saw with the Indignados was the first manifestation of a political crisis in our country. That political crisis is what explains the success of Podemos," said sociologist Lluis Orriols, professor at Madrid's Carlos III University. "Now we have an unprecedented generational conflict."
Read more: