A “winning candidate” is one who wins. It has not been demonstrably the case in recent years that a purely “moderate candidate” can be elected anywhere!
Scott Brown comes to mind, although there is really no such thing as a "purely" moderate candidate (just as there is no such thing as a "purely" conservative candidate).
I assume it takes a certain amount of political courage for a candidate in DE to even call themselves a Republican. In this case, we are talking about a seat held by Joe Biden for 36 years.