From what I have read, it happened in the late 1920's and early 1930's. In 1928 the New York Times published an article outlining the horrors being committed in the Soviet Union. The Times sat on this information for a while because they were liberal back then.
When it came out, soon other articles followed. Then news came out of Nazi Germany of the bad things they were doing to people with mental handicaps (they didn't start killing Jews until after WWII started). The Progressives of that era panicked because they had tried to convince America that Hitler, Stalin and Mussolini were "the men of the future". They took the name liberal because America was founded on what is now called classical liberalism and they wanted to be seen as perpetuating classical liberalism, when they were the polar opposite.
They went underground and stopped holding rallies (at places like Madison Square Garden) in favor of Hitler and the others. Many of them fled and dumped Marxism altogether once they saw the truth of what it is. Others tried to justify what Hitler was doing by saying socialism would work, Hitler just did it wrong. Still others just pushed Nazism and fascism off on the right, claiming it is a extreme right wing type of government. And it was the last one that stuck.
The idiots never took into consideration that those who didn't get caught in this moronic Marxist cult still believed in the what this nation was founded on and this nation was NOT founded on anything close to resembling Nazism or fascism. It was founded on a small, limited government where the individual was sovereign and held all political power. The people on the right (mostly) still believe in this America. They were never Nazis or fascists and their grandparents fought a war against those idiots.
But when you try to explain that to the left today they come up with stuff about how Hitler killed all of the socialists in Germany. So. Socialists have always been violent. German socialists had a few political differences with the National Socialist German Workers Party and because of that Hitler (who had absolutely no problem with killing people) just had the German Socialists exterminated.
In Italy the Italian Socialists were pissed at the fascists because they were a little different. So they murdered Mussolini.