Neil Kinnock was generally expected to win the British General Election back in 1992, but he didn't.

All polls are subject to things such as, what is the size of turn-out, and does this have a particular impact one way and another on the fortunes of particular parties. In this case the vote was a great deal higher than had been expected, as was inevitable on the morrow of the largest street rally in the history of the country.

Pretty obviously the bombings were a factor in deciding this election, but predicting which way it would have worked in advance would have been remarkably difficult. And one thing that needs to be taken into the calculation is the effect of the way the previous Spoansh Government responded, which was seen by many people as a dishonest and distasteful attempt to make use the bombing to get the result it wanted.

The other thing to have in mind is, "What is the impact that the bombers would hope to achieve in any particular election?" I would think that, in the case of the November Election in the USA, they would be hoping to have Bush elected, and I fear that a spectacular atrocity near polling day would be seen as a way to help that along.