Mr. Zapatero decided in April that he would not seek a third term. Instead, Alfredo Pérez Rubalcaba, 60, a PSOE veteran who was until recently the interior minister, will be the party’s candidate for prime minister.
Although Spain’s next general election was scheduled for March 2012, Mr. Zapatero said he had moved the date “to project economic and political certainty.”
The defeat of the Socialists in regional and local elections in May left Spain with a weakened government at a time of renewed concern about the country’s public finances. There is also broader uncertainty about the euro and efforts to rescue Greece and other ailing economies among the 17 countries that use the euro.
Mr. Zapatero’s decision was welcomed by Mariano Rajoy, the leader of the Popular Party, the main center-right opposition. Mr. Rajoy said it was “what most Spaniards had wanted for a long time.”
Mr. Rajoy, 56, was defeated by Mr. Zapatero in 2004 and 2008. While Mr. Rajoy’s own popularity has remained below that of his party, he is still expected to win in November after his party’s victories in the May elections. The Popular Party started overtaking the PSOE in opinion polls as Spain’s economic problems deepened early last year, after the collapse of its construction sector and amid a surge in the unemployment rate to 21 percent.