I am not familiar with that context, because I've never experienced the US election. But I guess a measure (we call that “项”）is a supporting point under a proposition (we call that "款"), just like in the text of a contract or in some other legal documents. On the other hand, these two words have different meanings, not actually the same. "Proposition" means something proposed by some one to be done, or a goal to met; whereas "measure" means some concret steps to be taken to hit that goal. Put simple, a "proposition" is "bigger" than a "measure."
