His gains were widespread, so explanations should start with the broadest factors -- not with bespoke stories about states, cities, counties, and groups.
The simplest explanation: party of unpopular incumbent loses.
Like clockwork, commentators interpret elections as ideological mandates. Trump wins, so voters hate progressives and want conservatism.
Here's why that's wrong.
1) It doesn't help explain why parties *of all ideologies* lost vote share post-COVID.
2) It exaggerates how much voters vote based on policy. So much polisci shows how difficult that is, and how in fact it's often the reverse: voters choose polices based on whatever their preferred candidate is advocating.
3) The big LOL comes when the new party takes power and goes in the direction that voters allegedly "wanted." And then voters...go in the opposite direction! This is the thermostatic model of public opinion.
Here's the graph. First, the "gold standard" ANES shows no secular decline in black partisanship. All of the decline is in the CES.
Moreover, the % Democratic is routinely higher in the ANES than the CES, especially if you include leaners.
Now, the CES has 3 more years of data (2021-23) than the ANES, which stops in 2020. It's possible that the next ANES (2024) will show a decline in Democratic partisanship among Black Americans.
Here's what I think would move the popularism debate along. We need some specific testable claims, because I see different versions of the idea circulating.
1) The popularity of the president's public positions is associated with their approval rating and reelection.
2) The popularity of positions associated with a political party (fairly or not) affect the popularity of all of a party's officeholders.
(e.g., this is the claim implicit in the idea that "defunding the police" cost Democrats in 2020)
3) The popularity of the positions associated with a president (or his party?) affect how many seats that president's party loses in a midterm election.
And does this apply to the out-party too? Do the GOP's unpopular positions matter right now?