In the late 1800's the 'Progressives' were Republicans. It was in the early 1900's that leftist California governor Hiram Johnson succeeded in turning this nations political process on its head - moving political power from the people into the hands of sitting government. Few today have the least knowledge of just how much 'the people' lost as the policies that Johnson initiated in California spread across this nation during the 'Progressive' era.
The Progressives were soundly thrown out of office nationally - and they migrated to the Democrat Party, where the majority have resided to this day. However, there are many conservatives that have remained in the party of the Donkey - hoping to take it back, or to restrain it.
You said it. I am also an avid history buff and I totally agree.