Actually, I must respectfully disagree with you on this point.

FDR was very much aware {as the everyday American was not} of the danger that a united Nazi Europe would pose to the rest of the world. Public opinion would not have allowed FDR to openly intervene in the British/German war {which is why he was forced to use indirect methods such as lend lease in the first place}, He would be booted out of office summarily for asking the US congress for a DoW against a nation that had just declared war on Japan for their sneak attack on pearl harbor!

Picture it:
FDR's speech to the nation... "Yesterday, Dec 7 1941, a day that will live in Infamy..." I ask congress for a DoW against the Empire of Japan, and oh yea, how about a DoW against the Germans {who just made a DoW against those who attacked us}, Right? Too be followed immediately by thunderous demands for impeachment, ect...

Welcome too the forums, by the way.