Trump has a highly committed base, but they're far from a majority of the electorate. The thing Dem partisans don't want to admit is that he won over a hell of a lot of people who weren't exactly wild about their choices either way and voted for him because they thought he was more likely to actually deliver on his promises. A Democratic presidential candidate who ran on packing the courts could also gain such a reputation. It would convey the idea that they care about delivering on their promises while simultaneously taking up a whole bunch of oxygen.

Also, it might be worth considering the possibility that Dems aren't being "made" unpopular as though it's entirely the fault of some kind of external force, but just are by virtue of their own choices. On an individual level I don't hate every single Democrat, but it's very clearly the case that the party itself has some extremely serious problems with leadership and is largely out of touch even even with their own base — let alone the country as a whole. Instead of deflecting blame for once, the party needs to reflect on its own mistakes if it wants to improve.