"So even for Bush and those very American republicans... It's just about the politics?"
It' more than just politics - the President cannot, in good conscience do anything that would undermine the war effort and an admission that he made decision long before he has said would really raise a lot of needless questions about why.
Frankly, Rockefeller is harming the war effort by bringing it up this much.
Sounds damned if you do, damned if you don't, and not just politically. In order to not undermine the war effort, Bush needs to sit quietly as others do?