Voting. The SCOTUS justices that the Republicans have put on the court seem determined to deny Dems the right to vote. Ever since Roberts, the most moderate member (which does not mean much) of the GOP Justices, helped to gut the Voting Rights Act, the court has stood in the way of the right to vote. With a few exceptions, they have ruled in ways that make it harder for the Democrats to compete.
While Dems can be divided on all kinds of issues--not all are feverently pro-choice, some are ambivalent about health care mandates, etc--one thing they can agree on is that they need to have a fair battlefield every November. That they are united when the GOP justices rule in ways that disenfranchise voters, in ways that limit the counting of ballots.
So, if there is one issue that makes court packing not just legitimate but necessary, it is the vote. Folks can argue about precedents and races to the bottom, but the GOP has already gotten to the bottom. There is nothing lower in a democracy (other than genocide and the like) than making it harder for people to vote, for the votes to be counted. There is nothing more basic to democracy than that. One party has been determined to "compete" by making it harder for people to vote. They have now created a court that supports that stance. THIS HAS TO BE UN-DONE. It cannot stand if we want real competition, if we want democracy to continue. Indeed, if the court keeps on helping block people's ability to vote, then the court will be illegitimate. The only way to save the court's legitimacy is to change its makeup.
So, yes, pass law that says that the US needs as many justices as are there are appellate courts (which is pretty precedented). That gets us to 13, as in 13 colonies. Isn't that nicely symbolic? And then the GOP justices will be outnumbered eight to five. With that many appointments, they can put on a mix of judges--not all flaming far left progressives, a Garland or two can be included.
Again, it comes down to this: the Supreme Court as it exists and will exist without any changes will continue to tilt the elections towards the Republicans by making it harder for people to vote. We can't allow that to continue. So, if given the chance, the Dems must pack the courts. Otherwise, kiss the courts and American democracy goodbye.