> the current political direction points squarely to Republicans getting absolutely pummelled in the midterms,

I am not this optimistic and I think it is dangerous to be this optimistic. Even putting aside mischief, just as a matter of reality, the gap always closes. A loss, yes. Hopefully. A slap around the face, maybe. Perhaps enough to get a clear win in the House. A pummelling? Nah. I think they might still keep the Senate.

Trump supporters falling out of love for him could well just lead them to focus their attention on down-ticket Republicans who figure out how to make them feel OK about their past choices, and since they need to feel OK, they will come out. Wholesale rejection of the party is unlikely since it is already splitting into two factions.