Nelson beat McCollum. There ain't no way he's going to lose to Harris. Florida is pretty much a 50-50 state on a state race level, as evidenced by the Bush squeaker in 2000 and the Martinez squeaker in 2004. If you're down 22 points in a state that has a 50-50 partisan split, you aren't doing very well.
Then explain Jeb stomping McBride despite a national effort by the Dems. (Hint: You can't)