Their study may be accurate for all I know, but "progressive" is a synonym for "leftist." It's in fact the term the Communist Party chose to represent themselves to the American public in the early 20th Century.
Can you back this up? Everything I've been able to dig up on this suggests you're wrong.
Specifically the
Progressive Party of the early 20th. Century was
not anything to do with the Communist Party.
The Communists called themselves Communists from the formation of the Communist Party Of the USA in 1919 onwards, and prior to that Socialists.
The Wikipedia article on
progressivism in the US says
modern progressivism is leftist, that of the early 20
th. Century was not.
Oops! Wrong Planet