I don't know how important the ETA issue is in the upcoming election, but it crossed my mind that this could be a "Reichstag fire" scenario by anti-ETA factions seeking to secure an anti-ETA government due to the backlash.Almost 200 people are dead. This would have to be followed by a dictatorial regime in order to keep the conspiracy quiet if you are right. Plus, the ETA has been very very inactive lately. The "anti-ETA" forces, if they can be referred to in this way (it'd be like talking about "anti-al Qaeda Americans") might benefit, but I don't see this as a real option for a Spanish political party.
Politically the ETA and Basque separatism are basically finished, actually.
This attack probably was the result of ETA desperation.
Very few Basques even support the ETA.