Well, in this case, I find that I slightly agree with Yeru, in that:
1) If the Gallup-style polls a week before the election were accurate, the Popular party was cruising to a victory...
2) If one assumes that a terror attack, regardless of who perpetrates it, two days before a national election, is intended to affect the outcome of the election...
Then whoever perpetrated the attack succeeded in affecting the outcome of the election.
Please note that the previous analysis does not require the attack to have been by al-qaeda. I am very skeptical about that, in that they ahve never taken responsibility so soon after an incident, and not so directly (I don't think they have taken responsibility for 9/11, except indirectly, for example - or the Bali bombings either). The fact that the "clues" were far more obvious than any other previous al-qaeda clues imply that it might not be them... but regardless, whoever did it succeeded in making people think that it could have been al-qaeda, and that's what made the difference in the election.
Please remember, everyone, that the incoming Spanish PM has only stated to not be part of the "coalition"... he has clearly stated that he will support a UN-led, multilateral force. Thats what the Spanish voters overwhelmingly have been polled as wanting for months and months...so in that sense, I guess Democracy works.