Tucker Carlson gave a good overview of why appointing U S Supreme Court justices is so important for Democrats. And why they unleash a slanderous vicious holy war on any Republican strict-contructionist/Constitutionalist nominee put forward by a Republican president. Not just by Trump, but by ANY Republican president over the last 30-plus years:

Tucker Carlson, Oct 12 2020, Monday:
https://www.youtube.com/watch?v=EWLpee8dcek

Democrats want ideologically partisan justices like Sotomayor, Kagan and Ginsburg, who will advance socialist/progressive reform through their court rulings, ignoring what the Constitution and past precedent say, to advance their Democrat/liberal agenda, as a way to shove their ideology down the American public's throat, that voters would never ask for or consent to.
And Democrat Senators and Representatives would be voted out of office if they tried.
So Democrats see a Democrat-partisan U S Supreme Court as a way to circumnavigate the system, and ram through their agenda on an unwilling public.

The last time Democrats rammed policy through the House and Senate (by breaking the rules, by the way) was Obamacare (a k a, the Affordable Care Act, or ACA). And most of the Democrats who legislated that were voted out of office in the next election, as a direct result. The people didn't like it. So Democrats prefer the more indirect and deceitful tactic of packing the court with justices who instead of making U S S C rulings based on precedent and the Consitution, instead impulsively rule based on things like
"a wise Latina woman with the richness of her experiences would more often than not reach a better conclusion than a white male who hasn't lived that life".