The problem with chosing election day is they aim to affect the elections, so they'll pull something first methinks. I'd think they'd go for the Spain effect. I'd bet they're hoping the Dems will try to impeach Bush. Depending on what they do, if it appears that Bush has not protected us here, it could easily turn the election on a dime.
That's also a possibility. In the case of Spain, they struck about 2 days before the election. That was time enough - by the evening of the attacks, the Socialists were in the streets, rioting in front of the PP (Partido Popular, the conservative party) headquarters and blaming Aznar. He was term limited out and was not actually running, but it was enough to defeat the PP candidate put up to replace him.
That time-frame worked so well that it wouldn't surprise me if they tried it again. And here as in Spain, they will have the eager cooperation of the left.