How Possible is this Map for the 2020 Election?

[SoLongAtlas]

Not possible in the slightest. Your map would make more sense pre-1960s with a strong and populist southern Dem on a ticket with a liberal Dem vp from the NE.

[SoLongAtlas]

Not possible in the slightest. Your map would make more sense pre-1960s with a strong and populist southern Dem on a ticket with a liberal Dem vp from the NE.

Then what is possible. Assuming that Trump's approval ratings sunk down to Bush or Truman levels, how much of a victory margin could there be?

It might be a 60-40 map, but it wouldn't look remotely like that.

How would it look? Where could I find the information so that I could assemble such a map?

Here's a baseline 2016 result map on wiki https://upload.wikimedia.org/wikipedia/commons/thumb/3/3f/United_States_presidential_election_results_by_county%2C_2016.svg/1280px-United_States_presidential_election_results_by_county%2C_2016.svg.png

You can edit the colors in paint or pixlr, etc. The key in 2020 will be suburb improvement with Dems, and flipping MI, WI, PA (upper MW) and NC, FL, maybe GA (South), and Ohio (central north). Focus on counties with high pop count or suburban counties with electorates that would trend Dem or have a lot of indy voters. Rural counties, unless won in bulk don't amount to enough to flip a purple state unless you max them out like Trump did white WWC and it remains to be seen how attached they will be to him in 2020.

This will help https://en.wikipedia.org/wiki/County_statistics_of_the_United_States as well as this site with cartograms, etc http://metrocosm.com/election-2016-map-3d/

[SoLongAtlas]

Still needs a little work but better for the goal of 60-40. The problem is that a lot of counties (read rural here) you shaded in blue won't vote for a Democrat under any circumstances, esp in the Deep South (and here it won't work even if you max out the black belt counties).

What I would do here to make it more "realistic" (in terms of predictions) is try to give 10 or so more percent to the baseline in counties surrounding cities and in counties trending blue or have a lot of indy registration. Having lived both in central VA and NoVA, for example, you just have to realize that Dems will not win in certain areas even if they ran with Jesus as the nominee to appeal to the right wing (most of the right wing would think it was a Mexican guy). Polarization is just too strong. Last Oct/Nov, I saw many houses in counties that you colored in blue with confederate flags and at least 5 Trump signs to the point where you couldn't even make out the front yard. My point here is that unless you have boots on, to use a mil phrase, in these areas and get a feeling of what is going on in those counties, you can't really do an accurate prediction analysis (that is what doomed Hillary), but not really making this that big of a deal since this isn't for polling, CNN, etc.

[SoLongAtlas]

Ok, here is a 2012-2016 comparison map I made to help out with a 2020 prediction.  
You can use that to get a visual reference on the swing/trend counties.

Note, even with Obama in 2012, he didn't max out counties that you have in your 60-40 map. He got 51.1%. So even with a relatively scandal free Romney (and he had his share of hits), polarization didn't allow for a 60-40 map. It may next time up but I doubt it breaks 55%.