[Poohbah]

Certainly the Senate has the lawful power to determine Senatorship...but is it spelled out in the law that no one else has that power under any other circumstance?

Yes. Basically, the Senate decides whether or not to seat the prospective Senator in question. Unfortunately, it's a one-time decision after each election.

[Southack]

but is it spelled out in the law that no one else has that power under any other circumstance?

"Yes. Basically, the Senate decides whether or not to seat the prospective Senator in question. Unfortunately, it's a one-time decision after each election."

I seem to not have that law in front of me. Could you show it to me, please?