When you have the media in your corner, like Clinton and 0bama, then you can pretty much do whatever you want. For the most part the media, other than faux and talk radio, hasn't touched f&f with a ten foot pole, only after Barry claimed EP did it become a major issue for the media, and they are doing their best to provide cover for that.
And the contrary for Nixon, when it comes to the media.
It all depends upon (a) who did it (and the media despised Nixon), (b) who was in control of things (and the media loved the Democrats in Congress), and (c) popular sentiment (the public being led by its nose by the media).
As you mentioned, Roosevelt, Kennedy, Carter, Clinton, and Barack Milhous got away with things (because the media adored, or adores, them), that Eisenhower, Johnson, Ford, Reagan, the first Bush, and the second Bush would've been lynched for, if they'd done the same things.
The internet's changing things, but not fast enough for me.