Pedro Sánchez told opposition parties in parliament that "when we will be forced back to the polls, I hope that the Spanish people give the Socialist Party an even bigger majority so that you ... cannot block the formation of a government that Spain needs."
He spoke a day after King Felipe VI announced that there was no viable candidate who could win parliament's endorsement before a Sept. 23 deadline.
Sánchez's Socialists won the April 28 election, but fell short of a majority and he was unable to win the support of any major rival parties. The far-left United We Can party both refused to enter into a coalition government or to endorse his formation of a single-party government. The center-right Citizens party also wouldn't negotiate until a last-minute effort that flopped.
Sánchez's rivals criticized his unwillingness to win them over.
Popular Party leader Pablo Casado, the conservative opposition leader, said that Sánchez "wanted new elections from the start, but you have spent five months playing with the Spanish people."
Early polls show the Socialists again winning the most votes, but still coming nowhere near the outright majority of 176 seats. Now they have 123 deputies in the 350-member lower chamber.
That means it is very likely the parties will have to change tact and find a way to strike a deal to get a government in place after the new vote.
Spain's caretaker government has limited powers and can't propose laws to the parliament.
The upcoming political campaign will likely coincide with a highly-anticipated verdict from the Supreme Court for a rebellion case against 12 of the leaders of Catalonia's 2017 secession attempt. A guilty verdict would likely spark protests from separatists in northeastern Catalonia.
Spain is also facing a slowing economy and, like the rest of Europe, the likelihood of further economic troubles caused by the planned exit of Britain from the European Union scheduled for Oct. 31.