Sanchez won the most votes in April's general election but he is far short of a parliamentary majority. He needs to negotiate extra votes to get lawmakers' approval, in a confidence vote later this month, to take office.
A fifth round of talks between Sánchez and We Can leader Pablo Iglesias broke down without agreement Tuesday. The two major right-of-center parties in parliament say they will block Sánchez's attempt to return to power.
The Socialists have 123 seats in the 350-seat Congress of Deputies, Spain's parliament.