A clearly disturbed and vile president rants about the conspiracies against him while contemplating suicide, and somehow is so full of pathos that we find ourselves feeling pity instead of anger.
There are...modern parallels? Robert Altman's Secret Honor's exploration of Nixon's psyche is a class of its own, due mostly to Philip Baker Hall's masterful performance. Still it does remind us of certain contemporary pieces, namely the first episode of Comedy Central's The President Show (particularly starting at about the 5 minute mark), and Aimee Mann's brilliantly tragic entry for 30 Days 30 Songs "Can't You Tell?".