Biden's approval rating is only 0.6% higher than Trump's at the same time period. He's not a popular president. The economy isn't doing well which has a very high impact on election results.
I think Democrats are taking a big risk going with Biden again, although I guess they would be taking a big risk changing him out too. What a mess, honestly.
Not irrelevant. Remember 2016? The options were even worse and we ended up with the cheeto. Since the DNC decided to stick with the senile sock puppet I would bet money we end up with Pence. And they will keep turning this country into a religious state. The DNC is fine with that too, because at least pence will play by the rules and keep everyone's doners happy. That is the only reason they hated Trump so badly. Had nothing to do with his actions, it was that he said the quiet parts out loud.
Never say never, but I don't think it matters all that much, with Trump in jail. But to answer your question, incumbent changing their running mate seems tantamount to admitting failure and you want people to view your administration as successful.
I asked a friend this the other day and they didn't know. In the last election Harris seemed to be set up to take over from Biden at the next election, but since then I've not heard anything about her or what she's been up to as VP. What happened?
Keep in mind that U.S. elections are not popular elections. You have to look at key swing states. Technically, none of the other 43~ states matter much because their outcomes are already statistically pre-determined. It doesn't matter how much people like Biden in California or New York, if he loses key states. For instance, Hillary Clinton had twice as many popular votes in California as Trump in 2016. But those extra 4 million extra votes didn't matter to the outcome because California is just one State.
For example, Michigan is a key electoral state. It is predominantly a red state. But Barak Obama won it two elections in a row because minorities from the densely populated areas, especially Wayne county, turned out for him. Those same minorities did not turn out when Hillary Clinton ran in 2016. They came back ... just barely ... when Biden ran with Harris as VP. This same story is repeated in Georgia and Wisconsin -- both swing states that usually lean heavily Red.
So, to sum up. It doesn't matter how much people in red states hate on Harris. Their election results have already been factored. What matters is how she is perceived in key swing states. While some red state voters won't vote for Biden because they fear he might keel over, other swing state voters will ... for exactly the same reason!
I think that no matter what, the younger voters will decide this time, but the old codgers that can barely walk better get up and do their share too.
It doesn't help that in some states they have made it very difficult for poor folks and minority neighborhoods to vote.