There's a misconception about Barack Obama as a former constitutional law professor. First of all, there are plenty of professors who are 'legal relativists.' They tend to view legal principles as relative to whatever they're trying to achieve.

Bill Clinton was impeached primarily for criminal conduct: lying under oath and misleading a federal grand jury about his affair with Monica Lewinsky. Nixon would have been impeached for a wide array of criminal acts, as well as abuses of power.