Just holding the deal together through a full four-year parliamentary term will be a tough challenge, many predict.
"The next legislative term will be very tough for the PSOE, it is likely it won't finish the whole term," said Catalan political analyst Joan Esculies.
Spaniards have also been called to show their ire in town square demonstrations across the country on Sunday.
For Andoni Ortuzar, the Basque Nationalist Party leader who also struck a deal with the Socialists on Friday, whether the government could hold would be a test of Spain's plurality.
"We are different - we consider ourselves a different nation - and the question is whether we can live together comfortably," he said.
Persons:
Sanchez, Pedro Sanchez, Francisco Franco, Junts, Carles Puigdemont, Joan Esculies, Alberto Nunez Feijoo, Andoni Ortuzar, Susana Vera, Emiliano Garcia, Puigdemont, Lluis Orriols, Ortuzar, Belen Carreno, Joan Faus, Emma Pinedo, Aislinn Laing, Alex Richardson
Organizations:
Deal, Spain's Socialists, Socialist, PSOE, People's Party, Socialists, Basque Nationalist Party, REUTERS, Catalan Socialist, Carlos III University, Thomson
Locations:
MADRID, BARCELONA, Catalan, Belgium, Catalonia, Spain, Madrid, Castilla, La Mancha, Spanish, Catalunya