Treat them how you would want to be treated if you were trying to learn about an apposing viewpoint. For example, when people use ‘libtard’, call the opposing side stupid for having their beliefs, etc, it might make them feel embarrassed and unwelcome if they had those views. Some might double down on their beliefs as a response in a way like, “Well screw you too.” There is an episode of South Park that I think does a pretty good job at showing some psychology about doubling down in anger. http://southpark.cc.com/full-episodes/s21e07-doubling-down#source=6154fc40-b7a3-4387-94cc-fc42fc47376e:fb76a7de-1c31-4186-abf9-2be2e3af4312&position=7&sort=!airdate

Anger comes from fear, so try not to make them afraid to join.

Finding points of agreement on policy and trying to focus on those is a good step too.

Also, before comparing the other side to Nazis, if you do this please ask yourself, “Are all of the people in the group you describe REALLY calling and working towards the mass murder of people based on their beliefs or state of being?” Please stay away from hyperbole, and be fair with the arguments if you want people to treat you and your arguments fairly.