Actually, from a tactical standpoint this would be a good move.
McCain continues moving to the middle, thereby narrowing Obama’s ‘target market’ to the kook-fringe left.
I’m not saying I agree with the move so put away the flame throwers.
This might be a "good" move for the short term single election.
But it is a bad move for the next election. He will be denying a Republican the VP stripe next time around. That is a RAT advantage.