Take a look at almost any state where Clinton underperformed Obama and you'll see that her numbers dropped more than Trump's numbers gained.
MI -- Trump gained from 45 to 47, but Clinton dropped from 54 to 47
WI -- Trump gained from 45 to 47, but Clinton dropped from 52 to 46 https://twitter.com/NilesGApol/status/1282762626796261379
MI -- Trump gained from 45 to 47, but Clinton dropped from 54 to 47
WI -- Trump gained from 45 to 47, but Clinton dropped from 52 to 46 https://twitter.com/NilesGApol/status/1282762626796261379
I can do more of this.
ND -- Trump gains from 58 to 62, but Clinton drops from 38 to 27 (!)
TN -- Trump gains from 59 to 61, but Clinton drops from 39 to 34
SC -- Trump stays put at 54, but Clinton drops from 44 to 40
MT -- Trump stays at 55, but Clinton drops from 42 to 35
ND -- Trump gains from 58 to 62, but Clinton drops from 38 to 27 (!)
TN -- Trump gains from 59 to 61, but Clinton drops from 39 to 34
SC -- Trump stays put at 54, but Clinton drops from 44 to 40
MT -- Trump stays at 55, but Clinton drops from 42 to 35
The only state where Clinton's numbers stayed put from 2012 but Trump's numbers dropped? Arizona.
And, dare I say, it's telling about Clinton's weakness as a candidate that in a state perfectly suited to reject Trump, all she could do was crawl from 44.45% to 44.58%.
What's my point? That in an environment looking to be significantly less third-party than 2016, all Biden really has to do is claw back the 3-4% Clinton dropped that caused her collapse in the rust belt. And as of now, he's more than doing that.