Weirdly, I'm actually more interested in seeing Fairfax win than I am in Northam, who I've never doubted. The Democrats desperately, desperately need young talent, and at 38, Fairfax will have a long future ahead of him if he wins.
In the end, though, I think they'll both run ahead of polls. If John Bel Edwards could run massively ahead of his end-of-cycle polling, so will Northam and Fairfax.
I think Herring definitely has a future, but Fairfax doesn't strike me as someone who will rise beyond the LG's office. OTOH, Virginia Democrats seem to be big on folks waiting in line and Fairfax is actually likely to be in better shape if he waits until after Herring runs for Governor (as I strongly suspect he [Herring] will in the next election), but I think Fairfax might be a bit too ambitious for his own good. I also worry that Fairfax is too liberal for where Virginia will be for the next few years, especially if we have a Democratic President in 2021 (another reason he's better off running for re-election as LG if he wins this year). Time is on Fairfax's side, but I don't know if he's smart enough to wait.
