Nixon and J. Edgar Hoover had complete disdain for our Constitution, and used their power to go after their perceived enemies.
They, at least, weren't narcissistic enough to broadcast the fact that they were going to do this; they tried to hide it, because even they knew it wasn't acceptable. Trump has made it clear - criticize me, and if I'm President, I'll go after you and make you pay. He's already banned multiple media sources from his rallies, because they're "unfair" - of course, anyone who disagrees with or criticizes him is "unfair" in his mind. From Generals to Justices and on down.
Can you possibly imagine the witch hunts Trump will start if he wins? A petulant, bullying child running our country, getting into Twitter wars, paying more attention to perceived slights to his ego instead of paying attention to our country. Presidents nowadays have more executive power, and too many voters seem to like that idea rather than understanding why it's so dangerous.
I was very young during the Nixon administration, and I like that he opened up discussions with China (although Obama was criticized for going to Cuba, which I agreed with for the same reasons as I did Nixon going to China) ... and pandas! ... but reading up on him and Hoover gives me the chills. Trump gives me far worse chills.