Which word is correct in this sentence: who or whom?

He said he would find whoever/whomever was responsible for the attack.

The trick I use is to substitute he and him. He = who. Him = whom. (If this sounds crazy to you, please hang in with me for a moment. It will get easier – I promise!)

He said he would find he was responsible for the attack.

He said he would find him was responsible for the attack.

I know, I know. It sounds like a crazy system! But it works like a charm. You knew he (who) was the correct choice. That means whoever is correct too.

He said he would find whoever was responsible for the attack.  CORRECT

But I promised you an easier way, and here it is. Whom and whomever are disappearing from our language. I say good riddance! They don’t add anything helpful to a sentence. It’s a pesky distinction that’s not worth the time and effort. 

Just use who and whoever, and you’ll be fine. That’s a guarantee.