Let me second what Big Media Matt says about Kerry's strategy to portray himself the candidate whose patriotism is unimpeachable. As he says, it may work in the primary but lord help us if he's the candidate and he truly believes that it's actually true.
My rule for this election - no candidate, by virtue of their resume, is immune from any kind of attack. Any campaign which clings to this fantasy is doomed.
With 5 seconds of thought I can think of about a half-dozen ways which Kerry is easily vulnerable along these lines - in some ways, perhaps, even more than the other candidates.
...Just to clarify, I wasn't making an argument that Kerry is therefore "less electable," just trying to get the point across that in this up-is-down world with a pliant RNC/Drudge talking point-repeating SCLM, even a candidate's "strengths" can easily be turned against him. That isn't to say that different candidates don't have genuine strengths, and of course they will and should try and use them, but no one should be fooled into thinking that they've been given an immunity card against anything based on those perceived strengths. A smart Kerry campaign will have already considered how to overturn the various smears I've anticipated, a dumb Kerry campaign will just assume that they can't happen. I'm not saying the Kerry campaign is dumb - this whole post was premised on "IF..."