I would also argue that this is clearly an example of partisan politics. There are several possibilities here:
A) the letter was sent because the Senators involved had no clue that this was clearly outside the bounds of their enumerated powers.
B) the letter was sent because the Senators involved set aside their otherwise strict adherence to the Constitution in order to advance what they feel is a more worthy cause. That would still reveal partisanship. This isn’t a life-or-death matter for the U.S. that needs to be acted on so quickly as to justify stepping outside the bounds of the Constitution. And this wouldn’t be much of a response if the Senators really felt that this was just such a dire situation, anyways.
C) the letter was sent because the Senators involved are willing to gamble on the issue, knowing that only one person has ever been tried and convicted under the Logan Act. Again, another obvious example of partisanship.
There is no way to justify the action. It is outside the bounds of the Senate’s powers, they clearly know it, and they acted anyways on a matter that, important as it is, is not of such immediate importance as to warrant such a maneuver. Why they’re doing it, I don’t know. But I know they aren’t doing it in adherence to the Constitution.
The only real debate is how serious a violation of their powers this is. Like I said before, I tend to disregard the Logan Act on this matter and head straight to the Constitution itself. In terms of the wider context here, it’s about on par with a wide array of moves from the executive branch that these very same Senators have lambasted the President for.