I don't accept their premise. I believe there are a lot of people who don't really like Trump but welcome someone who will shake up the establishment. So a significant portion of his support may be middle ground.
I had posted (back when Trump said to stop Muslim immigration until things can be sorted out) that there will be a massive push to make supporting Trump publicly unacceptable. They will attempt to cow people into hiding their support for him, avoiding bumper stickers and such, to downplay his popularity.
It worked in 2008 when they used the race card to suppress opposition to the Kenyan Muslim.
Trump has to be careful to avoid saying anything that would prevent the average Joe from being comfortable having a “Trump” bumper sticker on a car that he drives to work.