Dependent on regional parties for their lawmaking, the Socialists have, therefore, won a poisoned chalice in the Basque election. In recent days they have eagerly courted Catalan nationalist deputies from the north-east to make up for the missing Basques.
The government has at least 15 draft laws, covering everything from the promotion of renewable energy to the easing of abortion rules, that it wants to pass in the coming months.
"He doesn't have the numbers," one foreign ambassador says of Mr Zapatero, noting that the prime minister was finding the votes problematic even before the Basque election. "The number of bills, the number of hours in session are half those of last year because he doesn't want to go to the parliament."
José Blanco, secretary-general of the Socialist party, has put a brave face on the government's political problem, insisting that Mr Zapatero was installed without a formal pact among his supporters and that the party would continue to work day by day to achieve its economic and social aims.
"Let no one forget that the Spanish parliament, like Spanish society, is very diverse and there are many ways of operating, negotiating and reaching agreement," he said.
These ad hoc arrangements to reach parliamentary majorities – to pass the annual budget, for example – are known as the government's strategy of "variable geometry".
But leaders of the opposition PP, hostile to the Socialists almost everywhere except the Basque country and fresh from an outright victory in the Galician regional election, are looking not at geometry but at the simple arithmetic of parliamentary votes. They sense a political opportunity to humble the government after months of embarrassing scandals within their own party.
Mr Zapatero's opponents will focus on the economy, where the government has been persistently over-optimistic about the depth and duration of the recession, and moves towards further social liberalisation, including plans for a revised abortion law that have angered the Roman Catholic church.
Signs of government weakness are already apparent. Mariano Fernández Bermejo, the justice minister, was forced to quit after spending a hunting weekend (and failing to buy a hunting licence) with a senior judge who was investigating a PP-linked corruption scandal. Pedro Solbes, the finance minister, is expected to step down in another cabinet reshuffle soon, perhaps in April.
"Now is the time to talk of labour reform, of universities and schools, and of constitutional reform," says Joseba Arregi, a political commentator in Bilbao who is critical of Mr Zapatero. "Instead of the substance of politics, he gives us gestures, nice phrases and a smile."
If the past few weeks are any guide, Mr Zapatero will need more than his smile in the challenging months ahead.