It often doesn't require moderator intervention. Moderators will only see it if a [user has downvoted you over 5 times.][1] 

If someone has down voted you a bunch of times, a task runs every once and a whole by the system that will fix your reputation, and penalize the voter. This is known as "serial downvoting." 

Basically, the system takes care of this for the most part, not moderators 


  [1]: http://chat.stackexchange.com/transcript/message/3461515#3461515