They play the same game with Prez Bush. No matter what he does, it's wrong.
I thank war veterans whenever I see or even meet them. Once I was on my way back to my car at a shopping center when I saw an old man struggling to get out of his car and I spotted the "WWII Vet" license plate. When I approached him an asked whether he was indeed a WWII vet, his first reaction appeared to be to brace himself for a liberal verbal onslaught.
But when I said, "Thank you for a job well done," he smiled and said, "You're welcome."
It's simply called "moving the goalposts". GWB or the US military could find a cure for cancer and the left would in turn complain that the cure cost too much or doesn't cure everything, etc.
Using this method, the Left can compile a long list of percieved "failures" by their opposition without any effort on their behalf.