Spain's prime minister says he's open to a coalition with an anti-austerity party, hinting for the first time at a possible center-left governing alliance after Sunday's national election.

In an interview published Friday by El Pais newspaper, Socialist leader Pedro Sánchez says "it isn't a problem" for the far-left United We Can to become part of his Cabinet if he wins the tight race.

With Spain's electoral law banning polls during the last week of campaigning, it's unclear if the two parties will emerge strong enough in the lower house of parliament or whether a right-wing alliance could assemble a majority.

Sánchez is calling on Spaniards to cast a "useful vote" and has warned that the rise of the far right in polls could be underestimated given the large pool of undecided voters.