Acting Prime Minister Pedro Sanchez called the repeat polls after failing to secure support from other parties following an inconclusive election in April which saw his Socialist party win the most votes, but no working majority, in parliament.
Opinion polls however suggest this new election will fail to break the deadlock. Neither the left nor the right look likely to win a ruling majority in Spain's 350-seat parliament.
The Socialists look set to finish top again, but with slightly fewer seats than the 123 they picked up in April. The main opposition conservative Popular Party (PP) may also strengthen their parliamentary presence.
But the most striking development could be the rise of the far-right Vox party, which might even jump to third-largest in parliament, according to recent polling.
The Catalan question
The Catalan question is still very much in the political foreground. Less than a month ago, Spain's Supreme Court sentenced nine Catalan separatist leaders to lengthy jail terms over their role in a failed 2017 independence bid.
That triggered days of angry street protests in Barcelona and other Catalan cities that sometimes turned violent. More than 600 people were injured in the protests, which saw demonstrators torching barricades and throwing stones and Molotov cocktails at police.
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Sanchez said Madrid had sent a “significant” number of security forces to the northeastern region, to ensure the unrest did not disrupt voting in Catalonia.
During a TV election debate PP leader Pablo Casado called for a “real government that will put order in Catalonia”.
But the toughest line against the Catalan separatists has come from Vox leader Santiago Abascal. “Drastic solutions are needed,” he said during his final campaign rally on Friday night in Madrid.
He then repeated his pledge to end the Catalan crisis by suspending Catalonia's regional autonomy, banning separatist parties and arresting its regional president, Quim Torra, who has vowed to continue the secession drive. The crowd responded by chanting “Torra to the dungeon”.
At the rally, Ana Escobedo said she has voted for the PP in the past but was drawn to Vox because of its hard line on Catalonia as well as illegal immigration. “I think we heed to take a heavy hand,” she said.
Vox won 24 seats in parliament in the last election in April, in the first significant showing by a far-right faction since Spain's return to democracy following the death in 1975 of dictator Francisco Franco. This time it could double that number, polls suggest.
In recent days, Sanchez has repeatedly raised the alarm about Vox's “aggressive ultra-rightwing” policies, warning the party would drag the country back to the dark days of Franco's dictatorship.
“Spain needs a progressive government that puts the brakes on far-right and a government that puts the brakes on the extremists also here in Catalonia, who are the separatists,” he said during his final campaign rally in Barcelona on Friday.
Spain has been caught in political paralysis since the election of December 2015 when far-left Podemos and business-friendly Ciudadanos entered parliament.
That put an end to decades of dominance of the two main parties, in the eurozone's fourth-largest economy: the PP and the Socialists.
But there is a risk Sunday's vote will only prolong the agony. With no single party able to secure the required 176 seats for a majority, the Socialists are likely to opt for a minority government, ING analyst Steven Trypsteen said.
“Voting intentions appear to have changed since the April election. But these changes will not make it easier to form a government, so the political situation is likely to remain difficult after this weekend's vote,” he added.
Voting stations open at 9:00 am (0800 GMT) and close at 8:00 pm, with results expected a few hours later.