Modern politics can be summed up this way: Republicans frequently refer to Ronald Reagan, while Democrats prefer to conjure memories of Franklin Roosevelt.

Roosevelt’s New Deal rescued the nation from the Great Depression with economic reforms and bold individual measures including Social Security. But it was FDR’s unfinished business 75 years ago that profoundly inspires Democrats now seeking the presidency.

In his State of the Union message on Jan. 11, 1944, Roosevelt outlined what...