There is a fairly good discussion of this (the bombings and their effect on the elections) over in this thread on Metafilter.

It's unfortunate how it all happened, certainly the loss of life sucks. What is hard to imagien is hoe the Spanish who already know that appeasement is useless thanks to their experiences with ETA would now cave in to the obvious attempts by Al Queda to dictate their national policy.

Pacifism won't work. It never has.