At some point after 1900, using any means short of ASB, have a left-of-centre third party coalesce in the United States. It doesn't need to be full-fledged socialist, but at least social democratic.
When I say 'third party', I mean they meet the following checklist:
~Has a small but stable presence in Congress and multiple state legislatures.
~Presidential candidates regularly garner 5% or more of the popular vote.
Are you prepared?