Suppose you hired a contractor to put on a new roof and he ended up creating a huge hole in it instead. The contractor simply denies that a hole exists and keeps telling you to relax that your new roof is coming along just fine. The other contractor in town drives by and says he can fix that hole in your roof. You ask him how and he says, "well, I'll have to take a look at it and see how much damage is done but I have years of experience and a lot of good workers and I can get the job done for you. I'll tell you one thing, that guy you've got working on it doesn't know what he's doing. The hole's getting bigger while we stand here looking at it."
[Bush Supporters] would pick the first contractor because they know his work. (And he's a blast to have a beer with at the end of the workday.) The second guy refused to say exactly what he would do without looking at the damage up close so he can't be trusted.
Pretty much sums up THAT argument.