The Socialist Party is ahead in polls on Sunday's election, but even in the most favorable scenario, Sánchez would need to form a governing coalition or at least get support from others in parliament to stay in office.
Citizens leader Albert Rivera has repeatedly ruled out an alliance with the Socialists, because Sánchez was elected prime minister 10 months ago with the votes of Catalan separatists. Sánchez responded on Tuesday at the beginning of a final televised debate. "It's not in my plans to make any deal with a party that has put a sanitary cordon to the Socialist Party," he said, a reference to the Citizens party's campaign urging a center-right victory to keep the Socialists out of power.
The Socialist leader has so far ignored an offer to form a coalition by the far-left Unidas Podemos (United We Can) party, which also backed Sánchez in parliament's no-confidence vote that ousted the previous conservative administration and put him in power.
During the debate on Antena 3, Sánchez instead repeated that he aims to win enough votes to lead a Cabinet with Socialist and independent members. Tuesday's debate, the second in two consecutive nights, was a key opportunity for the leaders of the top four parties to reach out to the nearly one-third of voters that polls say remain undecided. Those votes could swing the balance between the center-left and the center-right as each tries to reach the 176-seat majority in the 350-member lower house of parliament.
The Socialists and Unidas Podemos on the left hold a slight edge among the two-thirds of voters who have decided, but they still likely would be left to find support from smaller parties, including the separatists of northeastern Catalonia.
Facing them across the political spectrum, the once-dominant conservatives of the Popular Party are competing for voters on the right with the Citizens party and the surging national-populist Vox party.
Vox, which polls predict could get around 10% of the seats in the Congress of Deputies, was left out of the television debates after Spain's electoral board ruled that other smaller parties would also need to be invited. The party instead held an election rally in an affluent Madrid suburb.