Taking away the word "what" that turns it into a question, the basic sentence is "It does do good". subject = "it" verb = "does do" direct object and NOUN: "good" When you turn it into a question, the object becomes "what good", with "what" being an interrogative specifier.

Meaning : If Rubio is popular with whites and Cuban-Americans, how will this be an advantage to the group of political candidates of which he is a part?

- "good" = advantage/ benefit/ positive outcome - "do" = contribute to