The problem with the Kerry campaign is that a Democrat who is strong on national defense doesn't fit with the media script, so the media find ways to undermine him.
This isn't a paranoid conspiracy theory. It's a simple observation of how media work. There is a kind of meta-narrative that determines how candidates can be treated and which stories will be emphasized. No editor or journalist consciously makes this decision; it's part of the unconscious biases all Americans have, and news organizations are not immune.
In the corporate world, this is called "branding". If you are the low-cost option, don't try to transform yourself into the premium service. Wal-Mart isn't going to start selling fine French wines and haute couture anytime soon.
Hence we have Republicans, who are associated with hawkishness and martial valor, enjoying support in the War on Terror despite having a candidate and VP who avoided military service and whose actual successes fighting terror are few. It doesn't matter that the actual PEOPLE involved are weak. The Republican BRAND is strong in this respect, and that's what matters.
I constantly hear liberals saying "What is wrong with Americans? Are they stupid?" No, they're not stupid. They're biased. (And so are you! And so am I!) Americans have ingrained ideas about which party stands for what, and they're not going to abandon those ideas after one presidential administration.
Kerry needs to find a way to work within the narrative to convince people that he would be an effective terror warrior. Maybe he could use the fact that Democrats are seen by most people as smarter (or at least better educated) than Republicans. Take the angle that he would fight a smarter war on terror. He tried this once but unfortunately followed it up with "more sensitive", which was the wrong word to use, obviously, even though he didn't mean he'd be sensitive to terrorists.
The point is that trying to sell his military prowess is the wrong angle to take. It opens him up to baseless attacks on his valor, which will stick because of the media script "Democrats are draft dodgers and peaceniks". Which is exactly what happened.