If Herring wins however, Republicans can be theoretically considered strong favourites to pick up his senate seat; the Democratic elected base here is virtually non-existant and the GOP has some top-class candidates like Tom Rust who could run (especially if Rust loses his district, which is incidentally the bluest district within Herring's seat at around 60% Obama).
Herring's seat is 60% Obama and the district excludes the parts of Loudoun county that are toxic to Dems. Think of this seat as one that cobbles together the most Dem parts of Loudoun and also parts of Fairfax(where Dems are much stronger).