Nixon wasn't impeached but two impeachments have occurred that failed in conviction. Other than bribery and treason it depends on how the Senate interprets high crimes and misdemeanours as it's them who vote on conviction. At present it's made up of 54 Rep, 44 Dem, 2 Ind.
Nixon resigned because he was going to get impeached and not likely to have Congress save him.
But yes...the rest failed due to make up of Congress which I pointed to...
Tbf...it'll be all or nothing with Trump..if he somehow gets in it'll mean he'll made a fool of the pollsters which logically means also maintained a Republican Congress.
Although talking of the unconstitutional things he's suggesting even a SC balanced in his favour couldn't uphold his actions lol...so that separation of powers would save Clinton from Orange being the new black...