Not at all. They are just using a standard presidential campaign tactic of mentioning popular individuals as potential VP's in order to give their candidate an aura of centerism. They don't really want a McCain to run as their VP; what they want is to convince swing voters that Kerry is a moderate, neither liberal nor conservative, like McCain.
If the Democrats were worried about their base, they would instead be pushing, in a purely verbal sense, the possibility of a black VP. This year, their base is secure, so they can symbolically move to the center by talking up McCain.
Now, if Kerry wins, which I don't think will happen, McCain can, if he wishes, top off his career with a few years as Secretary of Defense.
A standard tactic? Who else used it? McCain isn't perceived as a centrist at all, merely an "independent".