Spanish Prime Minister Jose Luis Rodriguez Zapatero's Socialist party claimed victory in the country's general election Sunday, after preliminary results have indicated voters dismissed worries about a slumping economy, immigration and resurgent Basque militants to hand him a second term.

With 67 percent of Sunday's vote counted, Zapatero's Socialist party had 44.5 percent, versus 39.5 percent for the conservative Popular Party, according to the Interior Ministry. If the results hold, the Socialists were likely to gain a significant number of seats in the lower house of Parliament, but would fall short of an outright majority.

"We can say with clarity the Socialist party has won the elections," spokesman Jose Blanco told euphoric supporters at the party's Madrid headquarters. The prime minister "is in a better position to govern over the next four years and begin a new period of change and progress with a Socialist government. It is a great victory."

The results appeared to be a powerful endorsement of Zapatero's record, which included sweeping social changes, a withdrawal of Spanish troops from Iraq, and a drive to cede more power to Spain's semiautonomous regions.

For Mariano Rajoy, his conservative rival in Sunday's vote and in 2004, a second consecutive defeat would be crushing and likely increase pressure on him to step down as party chief.

Sunday's vote came two days after the killing of Socialist politician Isaias Carrasco at the hands of the Basque militant group ETA, which jolted Spaniards and prompted both parties to cancel final campaign appearances.

Some in Spain had predicted the killing might prompt a wave of sympathy and boost at the polls for Zapatero's party, especially after Carrasco's 20-year-old daughter Sandra made an emotional appeal Saturday for people to defy ETA by turning out en mass to vote.

The timing of the attack was reminiscent of an election-eve massacre by Islamic militants who killed 191 people in a string of bombings against commuter trains in Madrid on March 11, 2004.

Three days after that attack, Zapatero won a surprise victory amid a wave of voter outrage at the ruling conservatives, who blamed the attacks on ETA even as evidence of Islamic involvement mounted.

The tactics were widely seen as a bid to deflect perceptions that the killings were Al Qaeda's revenge for the government's deeply unpopular support of the U.S.-led invasion of Iraq.

Many conservatives consider Zapatero's 2004 victory a fluke, and saw Sunday's vote as their chance to correct it. Conversely, a strong showing by the prime minister would be vindication of his performance while in power and lend him a legitimacy that critics say he has lacked.

Sunday's results showed the Socialists on their way to winning 168 seats in the 350-seat lower house, up from the current 164 and just shy of the 176 seats needed for an outright majority. The Popular Party was also shown picking up seats, raising their total from 148 to 154. Both main parties gained at the expense of smaller leftist and regional parties.