Joe Biden is in serious, serious electoral trouble. 

In 2008, 131 million votes were cast.
In 2012, 129 million.
In 2016, 136 million.
In 2020, 158 million.

Joe Biden won the popular vote by ~7 million votes.

Turnout will look more like 2016 than 2020. Which means the vote total will likely regress by tens of millions of votes.

Of the 20 million voters who voted in 2020 but likely won't show up in 2024, how many are Biden voters? How many are Trump voters?

This, in a nutshell, is Biden's problem.