I would also say Gang of 14 hurt Dewine. Blackwell, no doubt, was unfairly lumped in with Taft. Dewine had one more bit of baggage.
DeWine wasn't able to stand for anything. Conservatives praised him for tax cuts and war support, but most everyone else saw him as a nobody. Brown came out fighting. Long-term, good riddance RINOs, but it's going to hurt having communists in office here.