I've staked out a rather unique position in the last few contested Democratic primaries, in that -- while having a preferred candidate -- I've strongly felt that many of the front-running contenders would make fine elected officials and I'd happily support the lot of them. This, of course, puts me in a decided-minority particularly in our post-2016 world which still sees the Democratic world divided between "neoliberal Shillary" and "Bernie was a traitor". And by all accounts, as we approach 2020, we're in for the same set of shenanigans.
Hence my endorsement of Scott Lemieux's views on "arbitrary dealbreakers" when assessing potential 2020 presidential candidates. It is perfectly fine to have a preference for one candidate over another, and it's perfectly fine to fairly call out a candidate for having weaknesses. If you think that Kamela Harris' record on criminal justice issues as California Attorney General was weak, it's entirely fair game to point that out. But what you can't legitimately do is come up with a hodgepodge of supposedly dispositive "standards" which fade into and out of existence as befits your preferred love or hate of a particular politician, or which decides that only weaknesses count. If Harris' weaknesses on criminal justice matters are your "dealbreaker", then you have to give due credit to Cory Booker for being strong at the issue -- to turn around on him and say "but Wall Street!" defines ad hoc (see also: it's more outrageous that Clinton supported the 1994 crime bill than that Sanders voted for it).
I'm not sure it's intentional that the candidates most frequently subjected to get this sort impossible purity test seem to be women and minorities (Harris, Booker, Deval Patrick, are mentioned in this post, and I've also seen it applied to Kirsten Gillibrand and, of course, Hillary Clinton), but it perhaps isn't quite coincidental either. In any event, I will continue to oppose it (no doubt in vain) for the next three years. As the 2020 field develops, there will be perfectly adequate grounds to favor certain candidates and discount others. But the desire to preemptively exclude everyone (or everyone but one anointed saint) as insufficiently pure is poisonous.