The Don: I wish I shared your optimism. I firmly believe that unless there are exceptional circumstances like an economic crash, Donald Trump being arrested or an amazing and inspirational Democratic Party candidate being found that the President has a better than even chance of being re-elected - if he chooses to stand.
I say this because although he "only" has 40% support, that 40% are highly motivated to go out and vote for him and his racist policies. They're also the 40% least likely to have their vote lost by gerrymandering and/or voter suppression. OTOH a significant proportion of the 45%+ who may be thinking of voting for the other candidate need to be inspired to get out and vote and/or risk losing their vote due to the aforementioned gerrymandering and/or voter suppression.
I therefore think that the GOP will likely have the Presidency until (at least) 2024 and will retain control of both House and Senate for most, if not all, of that period. Those elder judges aren't going to last for ever and so a 6:3 or 7:2 GOP-leaning SCOTUS is very much on the cards. They will make the laws for a generation so look for states to get to choose on matters such as:
Legality of abortion
Teaching evolution
Gay rights
"Separate but equal" treatment of the races
The role of religion in lawmaking
This will allow the majority of the US landmass (though a minority of the population) to slip back to the 1950's where they're clearly more comfortable. Indeed, it may be like an antebellum US where progress is made in the integrated, forward looking states and the backward looking segregationist states talk about things like charm and matters whilst sucking hard on the teat of the government handout machine.