Like the objective pronoun whom, whomever works only as an object of a verb or preposition. Everywhere else, the correct pronoun is whoever (which, like who, is a nominative pronoun). Think of it this way: Whoever acts, and whomever is acted upon.
Examples
Misuse of whomever is very common. For example, whomever is questionable in the following sentences because a nominative pronoun (i.e., one that acts) is called for:
Why not just have everyone fill out a purity exam and whomever hews closest to the conservative line wins? [Dallas Morning News]
He did say, however, that whomever holds the seat in the future must remain vigilant … [Berkshire Eagle]
Whomever made the ancient paint selected only the brightest of reds. [AZ Central]
In each of the above sentences, whomever should be changed to whoever.
And in the following examples, whomever works because an objective pronoun (i.e., one that is acted upon) is called for:
That is how some Republicans characterized President Obama’s latest attempt to raise taxes on whomever he means by “the rich.” [American Thinker]
But all of us still have a right to criticize whomever we choose. [Charleston Post and Courier]
Whomever the Sox hire, he must bring in a strong-minded pitching coach to deal with this staff. [Boston Globe]
