Question on who / whom

I understand the basic difference between who and whom.. e.g, "who played the games, and for whom was it played for?" is correct so far as I understand. I use the old trick of substituting the who or whom with he or him; "he" being "who", and "Him" being "Whom".

However, one thing really irritates me. What is correct in the following sentence? : "Who / Whom did you say was coming tonight?" On one hand, you could say "Him you said was coming tonight"; but on the other, you could take out the "did you say" part of sentence, leaving "He was coming tonight. So it can make sense either way, if you see what I mean? Do I, in the sentence above, use who or whom?

Thank you