The Midwest was a Progressive stronghold in the early 20th century, so I wouldn't quite say that the rural-urban divide was a major cause of the failure for more left wing parties to establish a true toehold by 1920. I think it would have been a factor post World War II, however.
Check out this page of the 1924 presidential election by county.
Edit: And 1912 is a pretty good representation also. I'd have to dig around for congressional results, since presidential elections can be driven by personalities more so than choosing a third party.
Check out this page of the 1924 presidential election by county.
Edit: And 1912 is a pretty good representation also. I'd have to dig around for congressional results, since presidential elections can be driven by personalities more so than choosing a third party.