I can't see the poll, so I can't see if it is in one state, or who ran it. But, 40% seems quite high. If accurate, (which I would find shocking...but then, I find a lot of things shocking in modern politics), then perhaps it is because...he's popular?
If so, then the question remains: why is he popular?
I know two types of Trump supporters, although I will add a third class of Trump voters who don't really support him.
1) The first group consists of those who loved him from the beginning and still do. Most of the people I encounter in this group are on the internet, and they (the ones I encounter) have high school diplomas, are Union workers or farmers. (Side note: I only personally know one person in this first group...a relative of mine who is a minority and who is gay...and very outspoken in favor of Trump. Had a few beers with him at a family event a while back. The guy is a high information voter, active in the GOP (actually ran for office at one point), and he is hysterically funny, and totally blows away your prejudices against this group.)
2) The second group consists of those who like his policies (strong foreign policy, support for conservative values, and support for the common man), but they would hate him normally because he is a jackass. But, they have strong sympathy for him for being wrongly persecuted for the past 5 years. They think that persecution continues to this day. They think this persecution is harming the constitution, and they think he can stop that harm. This is the little old lady group of trump supporters, and I know a few of them.
3) There is a third group, very similar to the second, but they only support Trump when they have no viable alternative, so to call them "Trump supporters" is a big stretch. They vote against him in primaries. They are mostly "Dem opposers" because they hate the devisive tactics and labels of the Left, the sexualization of youth, etc.
You guys can't do anything about the first group of people, and neither can GOP leaders. (You guys abandoned this group when you became global elitists willing to import illegals to take their jobs.)
You could have diminished the second group of people, but you totally revitalized them by raiding Mar-el-lago. So, probably too late to do anything about them.
For the third group: You Democrats need to stop funneling tens of millions of dollars to the MAGA candidates in their primary races, and maybe allow the GOP to fight them better. You guys are helping to keep MAGA alive, by funding. Stop that.