Maybe centrist isn't the right term, when I say centrist I mean someone not framed as being far to the left. In the US someone seen as a solid liberal has less of a chance of winning than someone seen as a solid conservative. We've seen this even with Obama, Clinton and Biden, as democrats, but not really very liberal in the grand scheme of things. There are more conservatives than liberals in the US, but there's that middle area where people can be swayed - as you pointed out with the case of many of the same people voting for both Obama and Trump. Although it feels like today the political climate is different than it was in 2008, and more people seem to be dug into one side or the other.The problem with centrists is that many people perceive them (often not unfairly) as being fake and only taking positions to appease various factions of the electorate. Conventional wisdom is that swing voters are reasonable people who shy away from extreme, ideological positions, and are inclined to support a candidate who holds similar views. In reality, swing voters are low propensity, uninformed, and often take extreme, but inconsistent positions on issues. They generally don't vote unless they're won over by a charismatic, exciting candidate. That's why they can vote for both Obama and Trump. I think the best the Dems can do is have a wide open primary and hope that someone unexpected catches fire, as (Bill) Clinton and Obama did in 1992 and 2008 respectively. They really need to avoid nominating someone who comes off as fake, focus group-tested, and establishment-picked. If candidates who look good on paper won, we'd have just finished the glorious two-term tenure of President Jeb Bush.
I agree that the candidate can't be someone who just sits in the middle for the sake of not offending someone, but also can't be seen as too liberal, I mean we're still talking the about the US.
Anyhow, I'm looking forward to the leadership race to see who stands out.