Powell argued emphatically, behind the scenes, against Cheney, Rumsfeld, Wolfowitz, et al, against the invasion of Iraq, but in the end was unable to convince Bush to see reason. After he lost, Powell, being the good soldier that he always has been, put on his public face stood behind the President's decision.
Despite being the public face behind making the case to invade Iraq, Powell retains alot of his popularity in this country. Americans have always seen him as a person of integrity, and worthy of respect. His favorability numbers are as high as any public figure in politics. He has broad appeal to a wide range of the voting public, whether Democrat, Republican, or Independent. He's not exactly someone that would be easy to demonize as a target for character assassination.