All Presidents shade the truth. Presidents lead governments, which as a subset of bureaucracies, depend upon euphemisms and spin to depict the reality they prefer for their audience, the electorate. Part of the beauty of a government bureaucracy is the ability to spread around responsibility in such a way that no one can ever be held personally responsible for decisions that produce poor results. This has been the "Way of the Bureaucracy" from time immemorial. It is likely that the first tax collector termed his collections "mutual protection services" so as to avoid acknowledging that he was using coercion to take what belonged to another for the use of himself and his fellow bureaucrats. It is never surprising when a government, which after all is the largest bureaucracy extant, calls things by names which do more to obfuscate than illuminate and it is only when a particular government takes euphemism to such an extreme degrees as to invite parody that the citizenry takes notice. When the President declaims in ways which invite questions about his veracity, he is treading a slippery and dangerous slope.
During the Vietnam War, as the conflict became less and less popular, the Johnson administration adopted more and more jarring ways to avoid accidentally telling the truth; their efforts eventually led to the "credibility gap." Ricard Nixon went so far as to commit and cover up felonies in his efforts to keep his activities hidden from the American people. Bill Clinton is rightly infamous for his parsing of language in ways meant to deceive. Clinton, Nixon, and LBJ, however, did not start their presidencies intending to deceive (at least no more than to the usual degree a new President intends to deceive.) Barack Obama, emerging as he does from a background deeply infused with modern academic thinking, is already at risk of developing his own "credibility gap" but the genesis of his credibility gap is quite different from his predecessors.
Lyndon Johnson's lies were more the result of wishful thinking than of overt and conscious fabrication. He believed in the prosecution of the Vietnam War, could not accept that our tactics needed to be changed, and believed those who told him that we were winning, even when our tactics precluded actually winning the war. His greatest lies were lies he told to himself and they cost him his Presidency.
Richard Nixon lied with full awareness of his lies; he was determined, like most liars, to avoid the consequences of his behavior. When caught, he was impeached and forced to resign in disgrace.
Bill Clinton had the most flexible definition of the truth and took full advantage of that flexibility. Yet, at bottom, his lies were the most tawdry. He lied to avoid admitting sexual transgressions and there is good reason to believe that he lied about so many other things in order to preserve an image of himself as he would wish to be seen; ie, Clinton lied to avoid shame and humiliation. His Presidency failed because of his lies.
Barack Obama's lies are of a different nature. Consider two such examples in which Obama and his administration are choosing to lie to the American people, both courtesy of Glenn Reynolds.