Firstly, I assume it is meant to say "anti-graft party".
What part of "to be brief" do you not understand? "To be" is an infinitive form of a verb, which follows "threatens". What definitions have you found for "brief" in a dictionary?
To be short. The honeymoon (the time when the party is liked by people) will not last very long. "anty" should be "anti" - against or not for graft in this case.