It is an incredibly challenging endeavor! Getting these roles are generally very difficult. Not all politicians have a dedicated speechwriter, and their speeches might be written by their Press Aide/Press Secretary, Chief of Staff, Assistant, etc. The best place to start would be as a Press Aid, working closely with the Press Secretary or Communications Director to learn the fundamentals of messaging. You will first start by doing background research and writing talking points for the politician, but, over time, you will start to develop a very acute understanding of their style and will learn to speak (and write) in their voice.