Kurt Campbell and Michael O'Hanlon point out that perceptions of which party is good for national security switch around over time, typically in response to events. The current dissilusionment with the wreck Bush has made of things, in other words, offers up a chance for Democrats to shift around the post-Vietnam perception that they're worse on these things for the long-term. But to seize advantage of the opportunity, Democrats need to try, and outline "the kind of idea-driven agenda, and confident preoccupation with matters of national security that has generally been conceded to the GOP in recent decades."
I'm not sure exactly what Campbell and O'Hanlon have in mind, but their general take on this quite right. I would particularly emphasize the confident preoccupation point. One of the GOP's great strengths on the politics of national security over the past five years has, I think, simply been confidence. They act like they expect to win national security debates, and that helps them to win them. Democrats, by contrast, have mostly looked very defensive, a trend that's waned somewhat but still persist to a remarkable degree. But at this point, absolutely everybody can tell that Bush's policies have been a disaster. The first step to securing public faith in a Democratic alternative is simply to say that confidently and without self-consciousness.