I expect most of you will be aware that pollsters are often partisan? Most of them are private companies, and many of them do work for corporations regarding branding information etc. With regard to Brexit, Trump's election etc, all of the support of pollsters still didn't change the resulting voter decisions. But pollsters can sway marginal seats, and they can make those who are not committed to winning an election give up or believe the task impossible, especially if a minority party sees no progress in spite of their best efforts when viewed through polls that are 'weighted' towards the parties of the status quo. In short, the only poll that counts is the real vote itself, and anyone trying to change anything from an established norm only needs to remember that.