preposition of. After you invert the clause (you support whoever/whomever) and substitute,
choose him. Therefore, whomever is the correct pronoun in this sentence.
United States – BUSPROG.BENG.GUFF.17.6.6 – DISC.BENG.GUFF.17.6.6
United States – BUSPROG.BENG.GUFF.17.6.7 – DISC.BENG.GUFF.17.6.7