Prime Minister Pedro Sanchez fears that the right wing, led by Pablo Casados's People's party, will form a winning coalition with the far-right Vox party.
Spain is heading to the polls this Sunday in one of the tightest contests in decades that has been dominated by the rise of the far-right.
They're the anti-immigrant Vox party whose support base lies in the southern region of Andalucia where most migrants enter Spain.
For now, they don't even have a seat in the national parliament, but in a few days, they could be part of the next government.
Left-wing Prime Minister Pedro Sanchez says he's scared that the right wing, led by Pablo Casados's People's party, will be able to form a winning coalition with the Vox party.
If it comes together such a coalition would be part of a broader far-right movement that has already entered government in some European countries, notably Italy.
"No one gave Trump any chance of becoming president and he is. No one thought (far-right Jair) Bolsonaro would become president of Brazil, and he has," Sanchez told a crowd of around 3,000 in the working class Madrid suburb of Vallecas.
The polls indicate that Sanchez might only survive if he joins forces with Catalonia's small nationalist parties.
During the election campaign, the right accused Sanchez of being a traitor as he steadfastly refused to discuss Catalonia.
Under the most optimistic scenario for the left, Sanchez could stay on as leader of the euro zone's fourth-largest economy with just the Podemos party as an ally.
Up to four in ten voters were still undecided in the last official opinion poll, taken on Monday. Since then, informal soundings by media organisations suggest voters' intentions may have shifted, with Vox the possible beneficiary.
That has raised the prospect of the rightist coalition, which would also include centre-right Ciudadanos, winning enough votes to form a majority — although many view a deadlocked parliament and fresh elections as the likeliest option.
The leader of Spain's far-left United We Can (Podemos) party, Pablo Iglesias cast his ballot early in Madrid. The former TV politics commentator, is trying to harness anger at austerity measures.