Everyone post-primary thought Gillum would win until he lost.
That isn't what I remember. I remember people saying he would lose until it became clear that polls were showing him ahead.
He was seen as a weak candidate during the primary, yes, but ultimately the polling average pretty much never had Democrats losing FL-Gov in the 2018 cycle until it actually happened.
A better comparison -- a candidate running for an open gubernatorial spot in a state trending towards their party with strong backing from national figures who is seen as too extreme for the state, trails in polling, and ultimately loses narrowly -- might be Stacey Abrams, actually.
