> Honest question: do you genuinely not think that there are ever groups of people with an ideology based on ignoring inconvenient facts in favor of their preferred agenda ?

Plenty of people disagree with you (and each other) about which groups of people have these characteristics.

> the obvious follow-up question is how exactly you can effectively debate someone who quite literally is opposed to the idea of rational debate

I'm unclear on how "ignoring inconvenient facts" is supposed to imply "opposition to the idea of rational debate". But my experience has been that both are common among the most active and respected Wikipedia editors and curators. Just try to get one to give any concrete standard for what it would take to start or stop considering a source valid for WP:RS purposes, and then try to hold them to that. The combination of RS inertia with WP:NOR is the primary thing enabling citogenesis (https://xkcd.com/978/).

> it's hard for me to imagine how much more convincing the evidence would need to be in order to conclude that one of the major political parties in the United States has long abandoned any semblance of good faith.

If you think this is only true of one of those parties, you're part of the problem.