I have the complete audio soundbite in front of me cued up, and although he initially stammered and said “no”, he actually did admit it in a roundabout way by redefining “spy” and suggesting that the quickest route to surveil the Russians was through the Trump campaign. That’s an admission of the basic facts.
Another way this harmed Trump, aside from the obvious fact they were spying for the Hillary campaign or other political reasons, was that any “informant” they installed in his campaign would be taking up space that could have been filled by an actual dedicated campaign worker. In other words, the mole would be leaving work for which he’s being paid undone while doing Clapper and Brennan’s work instead. The informant handicapped Trump’s game.