Depends on who the Dem is. I think Bernie could've probably done fairly well in West Virginia this year against a traditional GOP candidate. However, Romney won 62% in 2012 so it's just a very red state now.
How would a populist pro-coal pro-gun Democrat do against a Republican deficit hawk like Rand Paul?
About as well as Alison Grimes did against Mitch McConnell.
I think better than that, depending on how pro-coal they are and if they can the endorsement of the UMW as well as the respect of the coal companies themselves.