In general, Progressives favored direct democracy. The 17th Amendment was part of a general movement that resulted in state-level changes providing for referendum, recall, and initiative. Popular election of senators was sold as a product of the same impulse.
Another argument for popular election of senators was that some states had gone years on end without two US senators because bicameral state legislatures in which the two houses were controlled by different parties couldn’t agree on whom to elect.
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