dems should not count on the recent momentum.
gop voters, and closet gop independents, should never be relied on selecting superior candidates, as 2-time trumpdom has cemented forever. the world knows the gop can rally the most asinine reprobate for any office at any point in the future. and they are good at it.

it is any combination of gop disinterest, or core dem enthusiasm, that will swing things. and this will be highly correlated to true independents.
and that group does not have to be overwhelmingly large to cause a severe swing for any given election cycle, which is why we have the frenzy of election-restructuring to tip the scales (gerrymandering, voter repression, etc...)
hell, as everyone knows, it doesnt even have to be the most popular or get a majority, depending on the structuring of the race. (which is what frightens dems)

the search for candidates everyone sees as superior (or simply likes) is useless, because elections are all about the real and imagined contrasts. 'let's find dems that that gop likes' is futile.