I’ve said this before, but it’s probably time to say it again: “who” is well on its way to losing its case marking. That is, the objective case “whom” is fading, leaving us with “who” for both subjective and objective uses (like the pronouns “you,” “it” and “what”). In conversational speech, “whom” is already long gone, and in all but the most formal writing, at best “whom” is inconsistently used. Anymore, the only place you really need “whom” is directly following a preposition: To whom it may concern.
When I’m editing, I change an incorrect “whom” to a correct “who” far more often than the reverse. Hypercorrection – using “whom” where “who” is actually right – leads to sentences that are both stuffy and wrong. Here’s an example: “Please forward this message to whomever is in charge of purchasing.” The pronoun here is the subject of the relative clause and so should be who — you wouldn’t say “him is in charge of purchasing.”
Here’s a short video I made to explain this to my students:
Bottom line: When in doubt, go with “who.”