Acccording to Slugger O Toole and his readers The Spanish turned against their government because of misinformation and manipulation since the bombings. It is alleged that the Spanish press did not cover the ETA denials before the elections. Verifying this kind of information is perhaps difficult. How do we know if the writers on that blog are themselves misinforming us!
UPDATE- Since I am currently in Spain, I thought I would try and verify the information reflected in this post. In fact, the comment appears to itself be misinformation- at least in part. I have just been able to speak to a Frenchman who is living here in Spain who states that from the moment of the bombing his perception was that the anger we all saw on TV was in fact directed against the ruling government for becoming involved in Iraq and thus
provoking this attack. He also stated clearly that he had seen the ETA denial and the Morrocan arrests on Spanish TV well before the election.
Clearly though, both parties appear to have used the attack to gain political points, and the socialists were more effective at doing this.