Few political spectacles are more amusing than watching Democrats who are millionaires attempting to deny that they consort with other millionaires, much less with dastardly billionaires. This was on extended display at Thursday’s presidential debate, and it offers a lesson about money and politics.

South Bend Mayor Pete Buttigieg has been raising millions of dollars in Silicon Valley, New York, Hollywood and other well-to-do progressive enclaves. This has riled Elizabeth Warren, who used to be a favorite of the wealthy liberal...