Here is a letter from General President Schaitberger regarding the IAFF stance on the following topics, Your voting rights, candidate endorsement, Dues money used for political endorsement. and personal beliefs. This should help answer most of the memberships questions.
I want to make it clear that just because the IAFF office in Washington D.C. endorses a candidate does not mean that your local is endorsing them or giving any part of your dues money to them.