I mean, consider the facts. Kerry is a "flip-flopper" in two specific senses:
-- He often takes a fairly nuanced stance on issues. He tends not to be blindly for or against things; rather, he tends to take a very middle-ground position in most cases, and precisely what he says depends on precisely what he's asked.
-- He occasionally changes his mind. I can blame him a bit for not being willing to say that in as many words, but modern American politics are bizarrely harsh on people who do so. But y'know, I have a lot more respect for someone who can change his mind from time to time, instead of sticking to a decision through thick and thin, regardless of later-revealed facts.
In other words, "flip-flopper" is being used as code for "moderate". This is apparently contrasted with words like "bold" and "decisive", which are being used as code for "extremist". I mean, Bush is drawing a very true contrast here. Whereas Kerry generally strives for the political center, Bush is constantly pushing towards the extremes: his views are in every respect more right-wing than any President in my lifetime. (He makes Reagan look like a weak-kneed liberal by comparison.) Whereas Kerry is constantly re-examining the landscape, Bush never changes his mind about anything, irrationally clinging to every decision, no matter how bad it turns out to have been in retrospect. (At best, he occasionally rationalizes that he has completed what he set out to do, and doesn't need to do that any more.)
Is Kerry a flip-flopper? Yes, if you use this definition. But y'know, if you're going to use that definition, a little flip-flopping is exactly what the US could use right now. And it depresses me to realize that a lot of people simply don't have the critical thinking skills to decode what's really being said here...