Why are Kerry supporters implicitly conceding that Bush is stronger on terrorism?

I'm watching Howard Dean on Hardball, and he's walking a thin line trying to accuse Bush of raising the terror alert level for political advantage without actually accusing him of doing so. (Here's the story on raising the terror alert.) Dean is pointing to the age of some of the information relied on (though other information is from the last week or so) and the fact that it is coming out at a time when Bush has a motive to overshadow Kerry (though raising the terror alert right before or during the convention would have been more harmful to Kerry). Kerry is distancing himself from these accusations, to his credit. But using Dean's logic--look at what is happening now and who stands to benefit--why shouldn't I suspect that Kerry is sending out Dean as his attack dog?

But quite aside from all of that, a question I have is this: why is heightened concern about terrorism seen as something that advantages Bush? Why doesn't fear of terrorism make us want Kerry? Terrorism really is a huge problem, whether it's weighing on our minds on a particular day or not. If Kerry supporters are saying that worrying about terrorism helps Bush, then we should be for Bush!

I can understand that Kerry supporters want to do what they can to help him, and I understand why it can hurt Bush to make us believe that Bush is manipulating the terror warnings for political purposes. But why are they conceding that that people concerned about terrorism will rally around Bush? Why isn't the argument that Kerry will do a better job of dealing with terrorism?

UPDATE: The answer to my question has to be that Kerry supporters are simply accepting the polls that show that the voters think Bush is stronger on terrorism.