Whomever is rarely used in modern British English, more rarely even than whom. One reason for this may be uncertainty about its correctness.
Whoever is clearly correct in: Whoever wants to know the truth can ask me. (=any person, subject, who, subject)
Whomever can be justified in: I will hire whomever I can find. (= any person, object, whom, object)
However, which is correct in these? -
1) Who(m)ever we elect for president will serve a four years term. (=any person, subject, whom, object.
2) I will hire who(m)ever turns up. (=any person, object, who, subject).
Be like 99% of native speakers and don't use whomever.