It comes from how Gillum differs from Democrats who have run in the past. Normally, you wouldn't think that a diverse candidate who represents the left flank of the party is a strong general election candidate in a state like Florida.
If Gillum wins, he'd turn that conventional wisdom on its head.
Gillum sets a contrast with Republicans in a way that many electorally unsuccessful Democrats in Florida haven't.
Democrats have mostly tried to win Florida with more moderate candidates. They literally put up former a Republican in Charlie Crist to run for governor in 2014, for example.
Gillum clearly isn't in the more moderate camp with his calls for Medicare for all. A Gillum win would put a feather in the gap of progressives like Bernie Sanders heading into 2020 who think Democrats work best with a strong left message.
I'd love to see if Rubio ends up supporting DeSantis ( as his campaign backer did ). I get the feeling Rubio's hometown campaign gave a little more momentum to the red side.
Gillum was down in 4th in polls last I checked before early voting. People speculate Bernie's couple rallies gave Gillum the boost for the winning 2 percentage points.
I've been registered independent. I think I'm changing sides in order to support DSA-affiliated candidates in future primaries... as nice as voting local exclusively for school board and judges was.