The incumbent tends to get damaged by bad things happening, regardless whether he has actually done something wrong or not. Secondly, I read in the paper that Bush might be forced to go up against the meat industry, which could hurt him financially (or, if he doesn't, he could be hurt in the public opinion). But you are probably more updated than I am, I just read about it in my morning paper, and it claimed that it could possibly hurt Bush reelection chances, not dramatically of course. For the record, I am not suggesting that an outbreak of BSE will prevent Bush from winning. Just that it might be harmful, and the Dems need everything they can get.
Are you saying that mad cow disease will hurt bush's chances of re-election? I hope you're kidding.