In OTL FDR had privately indicated he did not intend to run again, unless the situation in Europe got much worse - may '40 changed that.
If the sitzkrieg is prolonged a few months, but otherwise proceeds more or less as OTL, it might have been too late for him to grab the Democratic nomination, but he could still run as an independant.
If that happens you may want to look very carefully at his VP choice, because the MUCH more active campaign could be deadly