I guess it depends on how much of a role negative partisanship actually plays. If the opposing candidates are the biggest motivator for turnout, then the Democrats might be smart to pick a centrist. Trump’s strategy will be name-calling and labeling, like before — trying to keep his base angry and afraid. You can’t stick many damaging names on most centrists, because you can’t caricature their positions. A centrist might come with the advantage of a lower Republican turnout, since the Trump voters would presumably be less angry than they would be at Warren or Sanders.