I would say that ideological fanaticism can function as an equivalent of mental illness

even when no medical dysfunction is present.

The right in general and Trump in particular have been demanding of their followers for a long time that they be willing to put dogma above data, to believe their ears rather than their eyes. After decades of inculcation into this mindset, in parallel with similar efforts by evangelical preachers to breed their own kind of purblind fanaticism, we have a large population of people who are willing to act entirely as if they are mentally ill, when they aren't.