I'm thinking about how Trump is running for re-election after impeachment - afaict that's somewhat novel. Nixon as you mentioned resigned under threat of impeachment.

I think maybe the bar for conviction in the Senate is too high in a way that is corrosive to presidential accountability. Both Trump and Clinton committed serious crimes without real consequence and it is disgraceful, regardless of politics. (note: I don't care about the sex, I care about the perjury and obstruction of justice)