I don’t know what the definition of rational discourse is, but I think part of the idea is that you try to argue without using ad hominem arguments. My experience has been that ad hominem arguments — interpreted broadly — are the most useful type of arguments in terms of understanding what the other person is really getting at. People come to conclusions based on their own psychology, not on anything to do with facts (to the extent that there is such a thing as facts), so why is it considered a virtue to pretend otherwise?
I know it’s considered rude to say “you’re just a global warming skeptic because you enjoy being contrarian” or “you want to scrap the social safety net because you want the poor to suffer” or “you just want to bomb these people because you get off on the idea of violence and force” but the truth is that’s probably a lot more accurate than “you arrived at this conclusion by a careful examination of the existing literature blah blah blah”.
Nowadays, in part because of the internets, people can just make up their own facts. You can find some country somewhere where they eliminated all social services and the economy boomed or where the temperature went down the last ten years or where a long period of bombings and shootings was followed by a period of peace and stability.
So why should anyone have to argue against the Estonian miracle or the Pax Pol Pot given the knowledge that someone who likes Randism or genocide or increased carbon emissions can probably find some example about how Randism or genocide or increased carbon emissions worked somewhere, given enough time and access to google?
I think this is where liberals screw up. Nobody — not even the most hardcore tote bagger — is that convinced by an Ezra Klein piece about CBO estimates given that they can find some Tyler Cowen piece about CBO estimates that says the opposite.