One thing that Democrats will need to make a big deal of is the way that the media has given McCain a free-ride.
Right now, McCain has fairly high favorability and low unfavorability ratings. Most voters probably haven't heard his positions criticized very harshly by anyone other than far-right commentators. A progressive critique of McCain will be much more effective than the far-right attacks on him have been.
Right now, McCain is "solidifying his base", but the base is a relatively minor worry. However much the conservatives drag their feet, they will vote for him. The idea Republican lose because their base deserts them is a myth invented by the right to keep politicians loyal. McCain will have to worry about the center, who will peel-off in droves when they figure out what he really stands for.