According to the main Spanish pollsters, which are barred from publishing surveys from Tuesday, PP would garner 131-151 seats in the 350-member lower house, falling short of an outright majority of 176.
Vox is neck and neck with Sumar, a new alliance of far-left groups that includes Podemos, the junior ruling coalition partner.
Sumar would fetch 25-39 seats, polls show.
Sumar and PSOE have said they intend to recreate the ruling coalition.
Reporting by Inti Landauro; Editing by Andrei Khalip and Mike HarrisonOur Standards: The Thomson Reuters Trust Principles.
Persons:
Vox, pollsters GAD3, Pedro Sanchez's, Alberto Nunez Feijoo, Sumar, Sanchez, Feijoo, Francisco Franco's, Inti Landauro, Andrei Khalip, Mike Harrison
Organizations:
People's Party, Sigma, Reuters Graphics Reuters Graphics, Pedro Sanchez's Spanish Socialist Workers ' Party, PSOE, PP, coy, Vox, Thomson
Locations:
MADRID, Spanish, Pedro Sanchez's Spanish, Brussels, Huesca, Sumar