I remember getting into political arguments that went nowhere at the time but resulted in me changing my mind years later. The people I argued with never knew about my change of heart. As far as they knew I was one of those people who get more entrenched in their beliefs.
What I'm getting at is that your arguments can hit home without looking like it. What you're seeing as getting defensive could just be the early stages for them changing their minds.
This can be especially true if someone's political beliefs are part of their identity. You don't make those kind of changes all at once.
So I'd say just argue in good faith, don't try to score points, provide food for thought if you can, and hope for the other person to eventually find their way to the truth.