Hamilton wrote of this in the Federalist Papers. Can’t recall where.
Basically, they can Impeach for anything they can get the public to go along with. Doesn’t matter if it breaks any Laws or not.
I used to joke back in the late 90’s that a President can be Impeached if enough of the People didn’t like his haircut.
I never saw Trump on the horizon when I said that
65 & 66 that i recall