He that is without sin among you, let him first cast a stone at her. John 8:7
And if you really want to go back, former President Nixon is the only US President after World War II to have been successfully impeached. Of course, he lost his Presidency. Well, I won't go on, but it would seem that people expect their Presidents to be saints in the first instance, and secondly, that they should have prescience - with the ability to anticipate AND take action today for any untoward events tomorrow (as in the Katrina incident). This is in spite of the fact that any President (or his/her significant other) who sees a clairvoyant puts his ability to govern in doubt.
Life is cruel to those who aspire to high office, especially those who are humans and lack predictive powers.