They didn't have a single populist in the beginning. They just had "Tea Party Republicans". Unfortunately, this new "flavor" of conservative was even more racist and unintelligent than the original recipe.

Their voters immediately gravitated towards them, because they were seen as relatable outsiders, whose biggest policy position was to do whatever it took to hurt "the left", and/or anyone who even suggested the idea of "working together" with them. (ie. RINOs)

Trump just eventually took advantage of this movement and declared himself its leader. But when it first started, he still considered himself a Democrat.