What did the populists call for that would tax people with higher earnings more heavily?

Graduated Income Tax. The Populists viewed the graduated income tax as a means to pry loose a portion of the tremendous wealth of the nation's most prosperous citizens. A "graduated" tax meant that the rate of taxation would increase as one's income increased. A step was made in this direction in the Wilson-Gorman Tariff of 1894 when a uniform tax was imposed, but that portion of the law was declared unconstitutional by the Supreme Court the following year. Authority to impose such taxation was granted to Congress under Amendment XVI in 1913.