"Johnson . . . [said] that “they oughtn't be doing this"
"This is treason.”
It was and is most definitely not treason,
It may have been a violation of the Logan Act - If so, what the Logan Act proscribes is not "treason"; it is inter-meddling (messing in),
It is also the case that the Logan Act is regarded by most (certainly by most if not all of those now claiming "treason") as a very vile act (in both senses of the word. It brings John Adams no credit, and is widely regarded as having cost him re-election. .
Treason is, and is only, what Article III, section 3 of the U. S. Constitution defines it as being:
"Treason against the United States shall consist only in levying war against them, or in adhering to their enemies, giving them aid and comfort."
Nixon was clearly not "levying war against" the United States.
He was not "adhering to their enemies, giving them aid and comfort."
Reckless, irresponsible, potentially horrifically damaging to the Republic and the common good tossings about of "treason" -- against Nixon in '68, the 47 Senators in their fortunately unsuccessful effort to prevent what is now a fait accompli (may they go to hell, but it is not treason) need to stop.