The Progressive Party was created in 1912, basically because the Republican Party denied Theodore Roosevelt their presidential nomination, even though he won more primaries than his main challenger President William Howard Taft. And the Republican Party was the center-right party in America and if anything the Democratic Party thanks to the Dixiecrats in the party, were even further right than the Republicans. And because of the Progressive Era and other factors, TR and others believed their was a need and a movement in America to create a Center-Left party that could compete with both Republicans and Democrats in America.
Teddy Roosevelt had already been President of the United States as a Republican from 1901-09 and was a Progressive President. And was big champion of the Progressive Era and was able to put a lot of his Square Deal into law that created the American regulatory state. He was a successful and popular President and served two terms and could have been reelected in 1908 had he simply ran for a third term which was legal back then, but decided not to run for a third term because he saw third terms for the President as dictatorial and wouldn’t be good for democracy.
Post-Roosevelt presidency is where you see the real radical in Teddy Roosevelt. As if being a Progressive in a Center-Right if not further Right country in the first decade of the 20th Century wasn’t radical enough. TR became less pragmatic and moved from being a Center-Left Progressive, to more of a Social Democrat or Democratic Socialist. Arguing that human welfare was more important than individual freedom, individualism, and property rights. This is the TR that Socialists like Senator Bernie Sanders and many others in America, love and admire. Where Republicans like Senator John McCain who is a Conservative Republican, as well as Democrats like Barack Obama, Bill Clinton and others, I believe prefer the President Theodore Roosevelt. I prefer the Progressive TR, over the Socialist TR as well.