Even as late as 1909 the progressives were whining that the Constitution is practically unchangeable.
While it is true that Woodrow Wilson cooked up the living and breathing constitution concept in 1908, it wouldn’t become seriously practiced until the New Deal era, and widely practiced until the 50s.
The first progressive President being Teddy Roosevelt, Republican.
Well Republican until he ran in 1912 on the Progressive Party ticket, better remembered as the Bull Moose Party.
Some other prominent socialist/progressives of that era were Helen Keller and Mark Twain. Bet that will surprise a lot of people.