Why is Franklin D. Roosevelt called FDR?

In a speech to the Democratic National Convention in Chicage, on 2 July 1932, FDR said in a speech to that group: "I pledge you, I pledge myself, to a new deal for the American people. Let us all here assembled constitute ourselves prophets of a new order of competence and of courage. This is more than a political campaign; it is a call to arms. Give me your help, not to win votes alone, but to win in this crusade to restore America to its own people." (Source: search "FDR New Deal Speech"