They clearly know Bush is in potentially big trouble over this AWOL thing, and after seeing a few Rethugs bloviate the last couple days (including Bush on MTP), it's clear what they've decided on as a counter: a classic switcheroo, to make it appear that anyone who raises the issue is
denigrating the National Guard. While it may well be valid to raise an eyebrow at someone jumping to the top of the list (no doubt thanks to their daddy's connections) instead of going to Nam, that's not a good path to votes; after all, lots of people did similar things back then, and lots of people are in the Guard now.
And the real point is not that he went into the Guard, but that he didn't even play by the rules once he went in. The angle, IMO, should be that he got special treatment all the way through (overlooking the AWOL stuff, "working it out" so he could go to Harvard Business School, etc.); treatment "regular folks" would not get. A response might be "National Guard duty is honourable; however, being derelict in that duty is not". Please, add your own suggestions...