The correct answer is "Elected leaders were capable of abusing their power."
For Lyndon B. Johnson, he abused the powers given to him in the Gulf of Tonkin Resolution passed by the United States Congress. His massive escalation in Vietnam despite the lack of US success shows how he abused his executive power.
For Richard Nixon, he tried to cover up a huge break in to the Democratic National Convention headquarters at the Watergate complex. He tried to use the excuse of "executive privilege" as a means to keep his recorded conversations from the American public.