The debate on Spanish public television and the second on Tuesday on a private broadcaster are seen as key in mobilizing nearly one-third of voters who polls say remain undecided. Prime Minister Pedro Sánchez's Socialist Party is leading voting predictions in its bid to stay in office but short of a majority to form a government alone. But he refused to say which party or parties he might consider for a governing coalition if the Socialists win the voting in Sunday's election.
The question was repeatedly raised by the leader of the far-left Unidas Podemos party, Pablo Iglesias, who 10 months ago backed Sánchez's party in a no-confidence motion that removed the administration of the conservative Popular Party and put the Socialists in power.
Sánchez said only that he would prefer for the Socialists to win an "ample majority" and then have some independent "prestigious" ministers in his Cabinet. He also criticized the center-right Citizens party for urging voters to elect a "sanitary cordon" to isolate the Socialist party, saying Citizens is seeking a right-wing government, even if that requires the backing of the far-right Vox party.
"The only alternative to the three right-wings is the Socialist party," Sánchez argued. He said a Popular Party-Citizens government would put "the extreme right at the wheels," referring to the political coalition that late last year ousted the Socialists from power in Spain's most populous region, Andalusia.
Vox, the surging national-populist party, was left out of the television debates after Spain's electoral board ruled that other smaller parties would also need to be invited in line with the parliament that emerged from the last elections.
Most polls predict the Socialists and Unidas Podemos will emerge with the biggest block of seats but fall short of the 176 needed for a majority in the 350-member lower house of parliament. If that proves accurate, the left-wing coalition would need to seek support from smaller parties, including Basque and Catalan nationalists.
Catalonia's attempt to secede from Spain in 2017 has opened the gravest political crisis in decades. The separatist defiance has also prompted the rise of Spanish nationalism, giving wings to Vox, the party that could get around 11% of the votes, according to a mid-March poll by CIS, the country's official polling institute.
The 90-minute debate allowed the four party leaders to share some of their proposals for boosting economic growth, modifying taxes and repairing the country's failing pension system. But it focused on testing the candidates' rhetoric abilities.
Albert Rivera of the Citizens party struck a surprise note by attacking his most likely partner in any conservative coalition government, reminding Pablo Casado how his Popular Party had been tainted by years of political scandals. In response, Casado said he was chosen as his party's president "to turn a page on what could have happened in past times."
Nearly 37 million Spaniards are eligible to vote in Sunday's election, the third in less than four years as a result of the recent fragmentation and polarization of politics in Spain.