Given Trump’s tacking right, Biden should run his campaign as conservatively as possible. If he’s not ready to say conservative things, he should say nothing at all (and not just because he’s Pretty bad at saying coherent things).
I think about a general election in our two party system as an effort by each candidate to get three groups to vote:
Group 1) your own extremists
Group 2) your own moderates
Group 3) the unaffiliated moderates between you and your opponent
Biden’s group 1) is the left wing; trumps group 1) is the right wing; their group 2 is each party’s mainstream voter, and they share group 3: the moderates between them.
In the primary, traditionally, politicians ignore group 3 Moderates and message somewhere between their group 1 and 2.
Then, after winning the primary they shift gears and attempt to win group 3 by basically “moving to the center.” This means that normally, every politician is more extreme while campaigning in the primary, and more moderate while campaigning in the general election.
But, the extremists of your side feel betrayed by this. And if your party later loses the general election, your extremists tend to blame it on their having been deflated when the candidate trended toward moderation.
If the candidate is too moderate, the extreme of the party will not be energized to work for the campaign and so they—and the groups of people who they might otherwise influence to vote—will sit out the election to some statistical degree. What reallly matters is whether this is like, .001% sitting out or 1% sitting out, and in which states.
In my anecdotal experience, people on the right in American are better at supporting their own imperfect candidate while people on the left are more likely to hold a grudge against candidates they see as having betrayed their leftest ideals. I think this might be because liberalism’s current goals require more coordinated activity (government programs to increase opportunity for the underprivileged for example) while conservative goals on average require less coordinated activity (the removal of programs/tax cuts)—so, its like trying to hire someone to tutor your kids (left goals) vs trying to hire someone to just keep your kids from punching each other in the face or falling down a manhole (right). In other words the left has bigger objectives so there are more policy disagreements and more ways to be disappointed with your candidate. This translates to more purity demands from the left than the right (the purity demands exist on the right but are at lower levels of specification: no new taxes! No new gun restrictions! These are tests against action, not in favor of action).
All of this is to say, one challenge for democrats is that they actually do have to stimulate their left wing in the general, to a greater degree than the republican candidate has to. Republicans can generally run moderate generals in my opinion and win—and then they can govern slightly right of where they run; democrats have to run slightly left of where they will probably end up governing in order to win. I felt Obama did this: he ran left of where he governed.
Trump is not taking advantage of the opportunity to run a moderate campaign, instead hyper energizing his furthest wing of the party.
In so doing, he’s energizing Biden’s wing to vote against him.
This means that Biden should choose a moderate VP (Kamala Harris, or even a man) and he can then win the moderate middle, while also winning the frustrated left wing vote who, in this particular case, are going to be so energized by trump that they will show up and vote against him.
In terms of pure strategy. I agree with you. And I hate admitting that as a Leftist. But I'm still a realist. Fight for something better. Work with what you have. Biden will save us from hell fire...by taking us back to just the precipice of it. I'm already thinking about a post-Biden world because 2020 is shot. Unless no one takes your advice or Trump proves to be the despot he seems to be. I also don't share the optimism of many on the left who think Bernie will have any influence post-election. So I'll just continue to scream into the void while making whatever sad yet practical choices I'm given.