I go around and around in circles in my mind trying to figure out how we combat this.
Ezra Klein's book Why We're Polarized is one of a few books to attempts to make sense of what is happening. Timothy Snyder and Anne Applebaum have also approached this from different angles. Cult experts like Steven Hassan and psychiatrists have tried to deal with this too. It all helps contribute to some kind of understanding.
Trump voters, and many other Republicans, make decisions that appear irrational to us, but from their perspective, this behavior is rational and in their self-interest. That's something that is difficult for most people to comprehend.
The collective, or cumulative, radicalization of the supporters and the RW media is self-perpetuating, and people like Klein believe it can only increase in our current political environment. The immediate positive feedback they give to each other makes it self-reinforcing. They shut any information that conflicts with their world view out.
From the perspective of the radicalized politician: there is nothing to be gained by obeying norms or behaving ethically in the current climate.
There is that most important question you pose: What do we do as a party to counter what they are doing?
The trap they've sprung is a trap they laid several decades ago.
I'm bookmarking your post because it asks so many of the questions that need to be asked and answered.