I was reading Bob Woodward's "The Last of the President's Men"; the Nixon-Trump parallels are eerie.
The mercurial personality, the mood swings, the incoherent speech, the meandering rants, the contempt for the free press, the obsession with leakers, the pettiness, the vindictiveness, the overall criminality and depravity of mind and lack of morality or ethics.
The keys differences being: Trump says/tweets in public what Nixon only said in private (though the White House tapes are a great record of his true mendacity), and Nixon was a Machiavellian schemer of the highest order, which served him well in foreign policy in particular - his one real passion as President. Trump, meanwhile, is more like Reagan without the eloquence, or Dubya without the folksiness, in terms of intellectual curiosity (or lack thereof). And the parallels with Reagan in terms of being an aging celebrity whose White House is often in turmoil, whose Cabinet is often at loggerheads, and whose own cognitive abilities are increasingly questionable as time goes on are almost too easy as comparison.
But ultimately, Nixon is the closest parallel to Trump here, and that is truly frightening.