What is likely to happen next?
In 2016, Spain suffered almost a year of political dissatisfaction after two inconclusive elections, which left four parties haggling over who should govern. Sunday's vote can produce a similar result, opening up a new chapter of political uncertainty and fragility.
The main parties, however, have broadly split into two blocs during the election campaign, providing some guidance on what kind of governing coalition could emerge. Casado, the leader of the Popular Party, has portrayed Ciudadanos as his ideal junior coalition partner, and is hoping to gain support from Vox. On the other hand, Unidas Podemos, the far-left party, has pledged to support Mr. Sánchez and his Socialist Party
Depending on the vote split, the biggest unknown is the Ciudadanos posttelection stance, which won its first seats in 2015 as a centrist party and came close to forming a coalition government with Mr. Sánchez and his Socialists. Since June, the leader of Ciudadanos, Albert Rivera, has been among the most outspoken critics of Mr. Sánchez and has tilted his party further towards the right, notably after the regional elections in Andalusia.
Mr. Sanchez could still try to revive negotiations with Mr. Rivera, however, if Sunday leaves the Socialist Party as a clear winner but short of a majority
In all likelihood, negotiations to form a national coalition government will overshadow the campaign for the next Spanish election, on May 26, when voters will choose municipal and regional governments and members of the European Parliament.