The Basque separatist group ETA is seen as the likely culprit behind the assassination of a former Socialist local council member, Isaias Carrasco on Friday, March 7.
"This is a vile, cowardly act worthy of all our condemnation, carried out by a group of murderers who will never succeed in breaking the will of Spanish democracy," socialist Interior Minister Alfredo Rubalcaba told reporters.
The shooting came on the last day of official campaigning before Spaniards go to the polls Sunday to choose their next government. Both the governing Socialist Party and the opposition Popular Party cancelled their final rallies Friday.
Socialist Prime Minister Jose Luis Rodriguez Zapatero currently leads the conservative Popular Party by about four percentage points in opinion polls.
Uncertain election outcome
Yet the shooting could change the election outcome.
Zapatero had controversially opened peace talks with ETA before abandoning them in December 2006 after ETA killed two people with a car bomb.
The killing in the Basque country is not likely to alter this election outcome, said Julian Santamaria, a politics professor at Madrid's Complutense University. It could increase voter turnout, Santamaria told Reuters.
"It might mean more people get out and vote on Sunday," he said.
Placing the blame
In the 2004 elections, Zapatero's socialists were trailing the Popular Party in the weeks leading up to the elections. Then, three days before the elections, train bombings during morning rush hour in Madrid killed 191 people.
The Popular Party blamed ETA although evidence from the beginning showed that radical Islamists were responsible. Spaniards were outraged and voted the Socialist Party into office.
But this year, Basque separatism had taken a back seat to issues such as the slowing economy and immigration.
ETA has killed 800 people
Isaias Carrasco,a former Socialist council member, was assassinated in front of his wife and young daughter outside his house in a small town in the Basque region.
ETA has killed more than 800 people in the past four decades in an attempt to gain independence for the Basque region in northern Spain. Opinion polls show that while many Basques are unhappy with their current status as an automous region, most don't want complete independence from Spain. And only a small fraction support ETA.
Zapatero has been accused by the right of being soft on Basque separatists. But no criticism was heard Friday.
"This is a day of mourning. We should all stand by the family of Isaias Carrasco and remain united, united against ETA," said Popular Party candidate Mariano Rajoy.