"Had Calhoun been advised by me, he would have been the most popular man in the United States"

Duff Green's quote suggests that he thinks he might have been an excellent consultant to John C. Calhoun, the seventh Vice President of the United States. Green suggests that if Calhoun had taken his guidance, he would have been more popular than he was. This might be translated as Green thinking that he had the knowledge and insight to help Calhoun end up being more successful and popular. It could also be translated as Green thinking that Calhoun was not taking the ideal suggestions from the people around him, and that if he had actually taken Green's guidance, he would have been more effective. Green's quote indicates that he had the capability to assist Calhoun become more popular and successful, and that Calhoun was not taking the best advice from individuals around him.