Campbell is IMO the more appropriate comparison too - Mandelson was an elected MP and later a Lord during Blair/Brown years, he wasn't a SPAD for hire like Cummings or Campbell. That said, the three of them worked very closely together - Blair, Mandelson and Campbell.
I'm thinking mainly of the perception of each was that they were a dislikable Machiavellian power behind the throne: https://youtu.be/IWKBe7L50VA