After the debate, Sanchez will face a vote of confidence which he must win with a majority of votes in the 350-seat lower house. The Socialists won 90 seats in the December 20 elections and face an uphill battle to convince other parties to support its leader.

The vote is expected to take place March 2 or March 3. If Sanchez loses it, he faces a second vote within 48 hours in which he will only need the most votes to win.

The ruling conservative Popular Party came first in the election with 123 seats but its leader, acting Prime Minister Mariano Rajoy, refused to go for investiture, saying he lacked sufficient support to win either vote.

If Sanchez fails to win the votes, Parliament must choose a government in two months or face fresh elections.

The Socialist leader is aiming chiefly to win the backing of two newcomer parties, the far-left Podemos and its allies, which have 69 seats, and centrist Ciudadanos, which has 40.

However, both Podemos and Ciudadanos refuse to support a government that might result from any deal with the other.

Sanchez also faces fierce internal party opposition to a deal with Podemos, mainly because of its radical policies and insistence on allowing voters in the northeastern Catalonia region to hold a referendum on seceding from Spain.

The emergence of Podemos and Ciudadanos has led to Spain's most fragmented Parliament in decades and has shattered the dominance of the Popular Party and the Socialists, who have alternated running Spain for decades.