This post attributes a warning about European far-left radicalism as a threat to the U.S. to Senator Tom Cotton, but no independent confirmation of this specific statement was found. Cotton does regularly discuss left-wing ideology and has commented on European policy (particularly climate regulations), but the exact claim that he warned Europe's far-left radicalism is a "preview" for America cannot be verified. Without access to the linked image or confirmation of when/where Cotton made this statement, the claim remains unconfirmed.