The Madrid bomb attacks have injected a new element into Spain's weekend election and could swing voters depending on whether the attacks were the work of Basque guerrillas or Muslim militants.
If the Spanish government's initial suspicion Eta was behind the blasts is right, that could benefit Prime Minister Mr Jose Maria Aznar's ruling Popular Party (PP) which has campaigned on its tough line against the separatist group, analysts said.
If, however, some indications al-Qaeda could have been behind the attacks gain credence, many Spaniards might point a finger at the PP for stirring Muslim wrath by backing Washington and London in Iraq.
"Assuming it was Eta, the obvious emotional interpretation is this will make people back the party with the toughest line against them," politics professor Mr Josu Mezo said.
"If it was an Islamic extremist group like al Qaeda that carried out the attack, everything would change. But it is really impossible to predict at the moment."
Moreover, mass protests called by Mr Aznar for this evening under the slogan "With the Victims, With the Constitution, For the Defeat of Terrorism" were tinged with controversy.
The Basque Nationalist Party (PNV), which governs in the largely autonomous region, viewed the constitutional reference as unnecessary and politically motivated, given Basque and Catalan nationalists' desire for greater home rule or independence.
But the PNV said it would march under the pro-constitution banner.