1) Advocate 2) Advocator 3) An advocate of nullification 4) Exponent 5) Proponent 1) Invalidator 2) Negator 3) Not 4) Voider 
• (n.) One who nullifies or makes void; one who maintains the right to nullify a contract by one of the parties. • (n.) One who nullifies or makes void; one who maintains the right to nullify a contract by one of the parties.
Nul'li·fi`er noun One who nullifies or makes void; one who maintains the right to nullify a contract by one of the parties. 
[n] - an advocate of nullification
noun an advocate of nullification; someone who believes that a state can resist federal laws