Comey just confirmed both political parties were hacked in 2016 by Russians—but that only Democratic info was released.— Kyle Griffin (@kylegriffin1) March 20, 2017
Take a candidate we've been conditioned to dislike - for a good 25 years now - and just play up everything negative even more. Hammer on it night-n-day. It's especially effective when "the dirt" just happens to include the magic word "email", which of course links nicely to the more recent negative inferences about Hillary that were set in place over the last 5 years or so.
Eventually you can move the needle enough to make a difference.
Because advertising works. The world is being run by some pretty smart people (current POTUS notwithstanding), and smart people don't spend $500 Billion a year on shit that don't work.