A "Progressive" gets his name; or rather, originally got his name - because of the book "Progress and Poverty".
That is, this: A "Progressive" is someone who "is in favor of Progress and poverty" (The contents of the book; George's arguments, etc.) But they can't call themselves Progressandpovertyists, who are in favor of Progressandpovertyism. I feel silly even typing that, yet that's ultimately the most accurate way to describe it.
That's what a progressive is. That's how it started. Henry George. Progressivism is progressandpovertyism. Now, you can find plenty of small roots of the evolution of progressive from European authors, because the progressives here found European writing to be more "enlightened" than American writing, but this is a big root. This is the biggest root. Progress and Poverty is the tap root of progressivism. That's why so many early progressives were either directly Georgist themselves, or directly indoctrinated by a Georgist. In a tribute to Henry George, Fabian Socialist Bernard Shaw wrote the following:
When I was thus swept into the Great Socialist revival of 1883, I found that five-sixths of those who were swept in with me had been converted by Henry George. This fact would have been more widely acknowledged had it not been that it was not possible for us to stop where Henry George had stopped.
“George actually felt bound to attack the Socialism he had himself created; and the moment the antagonism was declared, and to be a Henry Georgite meant to be an anti-Socialist, some of the Socialists whom he had converted became ashamed of their origin, and concealed it
This presents the fork in the road.
In Europe and Britain in particular, the Georgist activists became socialist very quickly, as tyranny is how European culture is and socialism is tyranny. They had no reason to hide it. They had no reason to fight it. Culturally, it was a match set.
But in America, where socialism was viewed as a rotten set of ideals at the time, the big government types founded their own brand and called it "progressivism". This is why culturally the early progressives are different than the socialists even beyond the simple naming convention, but at the same time its why they(the progressives) had no problem mixing it up as friendly associates with the socialists. In general, they all had a common origin: Georgism. Shaw explains:
While others, including myself, had to fight hard against the Single Tax propaganda. But I am glad to say that I never denied or belittled our debt to Henry George.
Ultimately, the progressives and socialists started to merge back together in the 1930's, and then merged with the communists in the 1960's.
Culturally, American Progressives didn't initially believe in total government ownership of all means of production, they "merely" believed that government should regulate every aspect of business. Generically, the end result is indistinguishable yet culturally this difference must be explored as separate and distinct. That's why I treat progressivism differently than socialism. When Hillary or someone else says "I consider myself a modern progressive" as a way to deflect from them being a socialist, they are 100% lying at the same time they're 100% telling the truth.
They know you won't call Theodore Roosevelt a socialist. Nor should you. But in doing so, the modern progressive has absolutely disarmed the argument and won the discussion. It's a genuine historical paradox that exists only because historians don't tell the story of how progressivism came to be and how it continued to "make progress".
The original progressives weren't socialists, and I'll point to Theodore Roosevelt's own writings as well as the original Red Scare - That was Woodrow Wilson, folks. The progressives hated the socialists. That's historical fact. Today's progressives however, rely upon this relic of the past as if progressivism hasn't changed. It has changed, and today they are mostly socialist. But I still see no reason to let the original progressives get away with it. This a mythology, and we need to smash it. It's fragile, but its large. One person alone can't break it.
As long as the original progressives go unchallenged, today's progressives will continue to win.