A deal between Spain's center-right party Ciudadanos and the Socialist party could propel the latter's leader, Pedro Sanchez, to the role of prime minister next week.
Spain currently has a leadership vacuum following an inconclusive poll in December 2015.
The Popular Party, headed by acting leader Mariano Rajoy, took the most votes in that election but has failed in attempts to form a coalition.
The center-right Ciudadanos had talked with Rajoy but has instead chosen to form an alliance with his Socialist opponents.
"The agreement is intended for the vast majority of Spaniards, not for political parties," read a statement from Ciudadanos leader Albert Rivera on the party website.
Between them, the Socialists and Ciudadanos only command 130 seats of the 350-seat lower house.
And Sanchez will need further support to become prime minister in a confidence vote scheduled to take place on Wednesday, March 2.
"What we agreed we cannot do alone," said Rivera, urging other parties to join the alliance.