J Edgar Hoover had an informant(s) in each campaign and in various political organizations when he ran the FBI. I saw a young guy like me who had been frequently in YPSL/YSA events and then popped up in YFG/Goldwater events.
Unlike me, he totally changed his story when moving from one group to the other. (I kept the same story in all groups I was in.) Others in YPSL/YSA assumed he was FBI or Red Squad and would play practical jokes on him. I think he was FBI.
It is also quite common for Oppo Research to have an informant, and even a disrupter, in the campaign of the opponent. There is nothing worse than a trusted driver or non-policy person who totally messes up the schedule.
There was a Democrat mole who stole a Bush debate prep video.
Rolling Stone had a Democrat mole working on Bush’s campaign in Florida.
I can’t recall all of the outed moles the RATS have had in recent Republican campaigns.