Democrats admit they'd pack the Supreme Court if they win by increasing its size, destroying the nation's stability.
Democrat leaders are saying things like this: “If [the Senate] holds a vote in 2020, we pack the court in 2021. It’s that simple,” - Rep. Joe Kennedy (D-Mass), and no one in their party is really pushing back. Biden and Harris themselves refuse to say they won't, meaning they support the notion.
https://townhall.com/columnists/scottruesterholz/2020/10/01/biden-will-pack-the-supreme-court-n2577262
The last president to push for packing the court was FDR, and his own party shut that down as too extreme. No party has pushed the notion of court packing since. Until now.
Taking the radical step would render Trump's 3 picks irrelevant. It would also destroy the stability of the republic by rendering every pick irrelevant and the judicial branch itself a malleable tool of whatever party was in power at the moment. In a few decades we might have 150 people on the Supreme Court. It would become an absurd game. Not that the US would be likely to survive in its current form that long.
At best Democrats are guilty of partisan myopia. But the truth is that the left sees this election as their opportunity to finish off America once and for all, so destabilizing the nation is exactly what they want.