Acting Prime Minister Pedro Sanchez's Socialists said Wednesday they were confident of staying in power following July's inconclusive election, dismissing as "doomed" the conservative Popular Party's bid to form a government.

King Felipe VI last week nominated Popular Party (PP) leader Alberto Nunez Feijoo to try to form a new government ahead of a parliamentary investiture vote on September 27, even though he lacks support within the 350-seat assembly.

The PP won the vote with 137 seats and can count on the support of 33 lawmakers from far-right Vox as well as two votes from two small regional parties.

In total, that would give a PP-led coalition 172 votes -- four short of a governing majority.

"Feijoo's investiture is doomed to failure," Socialist party spokeswoman Pilar Alegria told reporters following a meeting between Sanchez and Feijoo.

"And once it fails... we will get an investiture that will bring stability to our country," she added.

If the PP fails to form a government, the king must pick a new candidate -- most likely Sanchez, whose party finished second.

If no candidate secures a majority within two months of the first investiture vote, new elections have to be called, probably in January.

Sanchez currently has the support of 164 lawmakers -- the 121 of his Socialist party, 31 from far-left formation Sumar, 11 from two Basque parties, and from the sole lawmaker of a small Galician party.

He is negotiating the support of two Catalan pro-independence parties which, if successful, would give him the backing of 178 lawmakers.

But the Catalan separatists have set the bar high for their support, demanding a referendum on Catalan independence and an amnesty for hundreds of people facing legal action for their role in a failed 2017 secession bid in the wealthy northeastern region.

During their talks on Wednesday, Feijoo asked Sanchez to be allowed to govern on his own for a two-year term during which the PP and the Socialists could work together to pass major bipartisan legislation on important issues.

The Socialist leader rejected the proposal, prompting Feijoo to tell a news conference that "Sanchez prefers to ally himself with separatists".