Answer:
It is as a slick politician that the Wizard relates to populism in "The Wizard of Oz."
Specifically, author and Oz series originator Lyman Frank Baum (May 15, 1856 - May 6, 1919) never left any known verbal or written indication that "The Wizard of Oz" was to be interpreted in a populist light instead. But for those who apply the populist analysis to the first book in the royal histories of Oz, the Wizard represents the slick politicians that achieve and maintain power on the backs of hard-working voters. So he stands for the kind of insincere, self-serving politician criticized by populism.