I don't see Bush's visit having any adverse direct effect on Nussle. But when a president comes into a state during the general election campaign, it can remind voters and sustain and reinforce the bad mood.
There's a point at which as president you can't game out everything. You've got to go to the core of your own way of doing things. This is a nominee, I would guess, in whom he has confidence and senses in his own mind that she has got the qualities for the job.