At home any form of socialist movement, including labour organisation, was suppressed to a degree that would seem incredible in Europe, which is one readon why today in the US there is effectively no left-wing.
Somewhere in Howard Zinn's A People's History of the United States is the figure for the number of activists shot and killed by hired strikebreakers, police and national guardsmen like those involved in the Ludlow Massacre of 1913, but at a quick look I can't find it. I'll try later.
(Zinn's book is one anyone interested in US history should read.)