Online comments are one of the worst places for mind-changing conversations. A key techniques to get around this: create your frame and question, but leave it open ended, requesting the reader to draw the conclusion. The example below is in response to someone who believes Sanders is a lost cause because he is too far left:
I think you're right that Sanders is much farther to the left than most Americans identify. But, if he were a left-extremist, we'd see him doing well with Democrats and poorly with independents in open primaries. It's completely the opposite. So something more complex is going on.
If I say "Sanders will win with moderates," the online answer is "no he won't." But even a reader inclined to disagree with me has to engage their thinking to think about what I think is complex.
Techniques from Appendix IV at the back of Cognitive Politics: start by agreeing as much as possible, set a frame but skip the conclusion.
Comment spark: In today's politics, what points are being pushed repetitively but ineffectively by your causes? How could you begin someone along the path that got you to your answer, without shoving the last step down their throat?