Bush's statement last Saturday that he ordered domestic spying, knowing it was possibly against the law to do so, was an astonishing confession in the annals of American history -- and the defining moment of Bush's tortured presidency. Why, after all, would the president open himself to the possibility, however remote, of an impeachment proceeding?
If American presidents stand for anything, it is deniability. This is the prime directive of presidential authority. More than 50 years ago, Franklin D. Roosevelt, who invented the Imperial Presidency and presided over the country during World War II, avoided leaving a paper trail of his most sensitive decisions. Even his order to build an atomic weapon was vague. "O.K. - FDR" is the only surviving record of his ever having granted authority for what turned out to be the most expensive and secretive project in American history.
Later presidents also took pains to make sure they were in a position to deny knowledge of executive actions clearly outside the law. President Eisenhower would not admit to flying spy planes over the Soviet Union at the height of Cold War hysteria in the late 1950s, even though his Democratic opponents were making false accusations about Russian capabilities to strike the U.S. with nuclear weapons. President Kennedy kept a safe distance from secret plots to kill Cuban leader Castro (unsuccessful) and South Vietnamese president Diem (successful). President Johnson approved the fabrication of evidence that led the U.S. Congress to authorize a wider war in Southeast Asia.
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Because rule of law is fundamental to the moral basis of the presidency, presidents must even uphold laws they don't agree with. Indeed, the willingness of presidents to do so is their defining trait. In this regard, presidents are unlike other citizens. They do not have the option to perform acts of civil disobedience. They cannot argue, in essence, that their conscience does not allow them to abide by the law.
http://www.alternet.org/story/29995/