I recently finished reading, The Ass Is A Poor Receptacle For The Head: Why Democrats Suck At Communication, And How They Could Improve written by Barry Eisler. It is a quick read that I would highly recommend and currently $0.99 on kindle. The essay resonated with me as I am have always felt that the left is awful at many aspects of messaging: specifically phrasing, sticking to, and not getting distracted from their message.
Mr. Eisler does an outstanding job of providing evidence as to why this is true. He also talks about what Democrats (and Republicans, though they seem to already understand these principles) could do differently to improve their message being received.
The author also talks about the difference between attacking the oppositions point versus the premise of an argument. A clear distinction that is not easy to accomplish in the moment, but when done effectively is the difference between blocking a thrust and countering it.
This essay was written in 2012, which means that there are many more examples from the last 8 years that could be added. As I reflect on recent movements that are associated with the left, I would postulate that they still have a lot to learn on the concept of messaging.
Read the essay yourself, and develop your own thoughts.