THE leaders of Podemos, Ciudadanos and the PSOE took part in a live online debate on Monday, but prime minister Mariano Rajoy declined to join the discussion.
The PP leader claimed he did not have time to attend the El Pais debate and had been invited to take in 30 debates so far during the election campaign.
The three others battled it out over corruption, Catalan independence and the economy.
It was the first time a general election debate had been broadcast live online instead of in front of a TV audience
Although heated at times, the debate was markedly different to previous years, with Iglesias and Sanchez both opting to not wear ties and all three candidates referring to each other by their first names.
Rajoy appeared in a simultaneous TV broadcast in which he pledged social security tax breaks for employes and employees.
Polls show his party holding a narrow lead going ahead of the December 20 general election, but look unlikely to secure an outright majority.