What will happen next?
In 2016, after two ambiguous elections, Spain suffered from a political weakness for almost a year, and four parties argued over who should govern. The vote on Sunday could lead to a similar result and open a new chapter of political uncertainty and fragility. To break the deadlock, repetition may be required.
The major parties, however, have largely split into two blocs during the election campaign that provide clues as to what kind of governing coalition could emerge.
The Catalan Front Party Esquerra Republicana is also committed to Mr. Sánchez The Social Democrats may hope to free themselves from another uncomfortable Catalan alliance.
Depending on the Votumssplit, the biggest unknown is the postelection stance of Ciudadanos, which gained its first seats as a centrist party in 2015 and approached a coalition government with Mr Sánchez and his socialists. The Ciudadanos leader, Albert Rivera, has been one of Sánchez's most vocal critics since June and has continued to turn his party to the right, especially after the regional elections in Andalusia.
Sanchez could still try to revive negotiations with Mr. Rivera when Sunday emerges as the clear winner of the Socialist Party, but the majority is missing.
In all likelihood, the negotiations to form a national coalition government will overshadow the campaign for Spain's next campaign elections, on May 26, when voters will elect city and regional governments as well as members of the European Parliament.