This question on politics.stackexchange.com isn't getting any answers, so I'll try a similar question here.
- Is there some statute or provision of the Constitution of the United States or case law saying that the way a U.S. senator resigns is by submitting a letter of resignation to the governor of the state that the senator represents, rather than to some other person such as the president of the senate (who is also the vice-president of the United States) or other federal officer?
- Is that considered the proper way to do it only because someone set a precedent and others follow precedent? If so, when was the precedent set? And might it be considered law if there is no Constitutional provision or statute or case law?