Cruz (Houston), Kasich (Lehman Bros), and Trump (NYC) weren't living in DC. Sanders seems pretty unlikely, as does Hillary.
I'd look at staffers and others closely affiliated with the five campaigns that were working or living in DC at the time - Bill?!?
They said it was a specific candidate, and not their staffs. Cruz also traveled lots to D.C. as SG, Kasich was a U.S. Representative and lived in D.C. for many years, and Trump traveled to D.C. on a weekly basis so it could have all been them. Hillary, unless she's just REALLY good at playing it straight, I think she's in the clear. Bernie, I mean come on. Nah. I think if it is anyone it's one of the Republicans, that seems to be her fetish anyways.
It's obviously Cruz. Trump doesn't need prostitutes, he can get any woman he wants. Same for Bill Clinton. Kasich and Sanders don't seem like the kind of guys who would be visiting prostitutes, plus Kasich was at Lehman Brothers in NYC which has better prostitutes than DC anyway.
Remember one of the two missing Cruz mistresses was a "high class DC prostitute" sounds about right.