One internal conflict democracies face is whether they should prioritize A) substantive commitments about justice or B) complying with the rules of competition and transfer or power.
Democratic voting is not like voting over, say, which vacation to take. If the family wants to (shudder) go on a cruise and I want to do anything else other than go on a godawful cruise, I might nevertheless (shudder) go on the damn cruise. After all, I value my family, and I recognize that we're talking here about personal preferences, not deep issues of justice.
But many issues in politics are about justice, not mere preference. We (at least many of us) tend to think there are independent, correct answers to political questions and that we should do what justice requires. Democratic decisions should track the truth. We recognize that any one of us could be mistaken when we think we've identified the truth, though we all tend to think we're better at tracking the truth than our political enemies are. (Or, more often, we think our team tries to track the truth while the other teams aren't even trying.)
When A (substantive justice) and B (procedural rules of competition and fair play) come into conflict, which should win?
You probably think that sometimes or even often substantive issues of justice trump playing by the rules. Sometimes the police officer should not enforce the law because doing so is unconscionable. Sometimes the jury should nullify the law. Perhaps a justice should bullshit a legal opinion because the point is to stop slavery, not to identity whether old legal cases and documents make slavery constitutional. Sometimes the senator should filibuster or game Robert's Rules to stop a declaration of war because the war is unjust.
Now consider the issue of filling a Supreme Court seat shortly before an election or waiting until afterward. If you think that serious matters of justice are at stake, and that waiting means that the other side will impose horrific injustice on the populace, you might well think it's more important to fill the seat now than to wait. On the other hand, if you think that the other side is committed to democratic norms and will not impose horrific injustice, and you recognize that the people have low confidence in your leadership, you might well think that it's important to wait until after the election, as this will imbue the decision with legitimacy.
A priori, there's no simple principle either way. It depends on what the situation is, what the stakes are, what will happen, and what the people think.
On this point, though, I think empirical work on voter behavior throws a real wrench in the whole issue. As Achen and Bartels say, elections are essential random events. If Biden wins, this doesn't show that the People genuinely support him or his politics. If Trump wins, the same goes. For 9 out of 10 voters, voting for a party or candidate does not demonstrate trust or belief in that candidate or party, and so there is little reason to think that their votes confer legitimacy. Regardless of who wins in November, the vast majority of voters will not have voted because they genuinely share their party's politics or because they genuinely believe their party is better. Once you study voter behavior, you can no longer say with a straight face that voting = the people speaking. So, it becomes far less obvious why we should wait. (That said, I hope Trump waits because he's an incompetent hack. I care about substantive justice.)