Trump didn't appear out of nowhere. Much of the bizarre garbage he's been spouting has been floating around the political commentary shows on Fox News for years. The difference is in how these conspiracy theories are approached.
Until now, the GOP was content to let Fox stoke public anger and then try to leverage that anger into votes. They wouldn't actually confront these "issues" because they know most of it is simply garbage.  They'd get blustery about it and then when elected, move on to real issues, while angry voters waited for someone to solve the imaginary problems Fox invented.
What Trump has done is take all these false, irrational conspiracy theories and turn them into election issues. By accepting these rants at face value, Trump has gained a following from the percentage of the public who accept everything on Fox as fact. While the GOP were content to play public paranoia for votes, Trump is actually building his policy based on that paranoia.
That is his core audience.