Bidens problem is people who voted for him?
To some extent, yes. There are plenty of people who voted for Biden, but are politically to the left of his agenda and are unhappy with him. That doesn’t mean that they’re ever going to cross the aisle and vote Republican, but rather may choose to either vote for a statistically insignificant third candidate, or simply not vote at all because they don’t feel there is a candidate who represents their interests.
Unlike Republicans, whose coalition is basically just the wealthy and the Evangelicals, the Democratic coalition is far more wide ranging. Also, the Republican coalition tends to be much more reliable about showing up to the polls EVERY single time. Given that the path to victory for Democrats is largely turnout based, having even a small portion of that coalition not show up because they’re uninterested could be disastrous.
I’ve been generally satisfied with the job Biden has done so far, but there’s no denying that A LOT of people who cast a vote for him were actually casting a vote AGAINST Trump. If/when Trump eventually starts to fade into the background, it’s going to be important for the Democrats to find a candidate that people truly want to vote FOR.