Well it's safe to say that it began as a social movement and grew into a political movement. The people that were part of this movement believed that the problems society faced (poverty, violence, greed, racism, class warfare) could best be addressed by providing good education, a safe environment, and an efficient workplace. As more and more people started to believe in the cause I reckon that it grew enough to engulf the whole nation. Some of the presidents during this time were Theodore Roosevelt, William Howard Taft and Woodrow Wilson.