It's long past time for George Bush to either find someone who's serious about winning this war or else someone who's serious about getting out. Rumsfeld is neither.
Rumsfeld is implementing Bush's policies. The idea that this is some sort of a technical mistake -- oh, we thought we could do with fewer soldiers, but we were wrong -- has long since lost any plausibility it might have had. It's not that Rumsfeld is neither serious about winning this war (putting aside the question of what precisely that might mean and whether or not it's still feasible if it ever was) nor about getting out. It's that Bush is serious about neither. These are Bush's policies, which he is pursuing as he wishes to. To blame them on Rumsfeld is to allow Bush to evade responsibility for his policies. It's just wrong -- factually, morally, strategically.
(This is not to deny that if Bush wanted to reverse course, firing Rumsfeld would be a sensible place to start -- if only for symbolic reasons, which in a war like Iraq might have a real effect. But until then, the responsibility should be placed squarely on Bush's shoulders.)
Or, as a different sensible liberal said (in a different context) today:
Republicans actually like the way things are going right now. The current state of the country is the Republican dream.
They're doing what they want to do. We need to avoid rhetoric that Bush is simply the victim of others' mistakes and incompetence. He has complete control of the government. If things are going wrong, the buck stops with him.