he actual fall of Great Britain will have an effect on the US public that we can only guess at.
Ape. Sh*t.
I would predict a suddenly angry public demanding to know how Roosevelt could have let this happen and Republicans and other semi-obstructionists posturing right along with the public, posing as always having been advocates for squishing the Nazi threat.
Roosevelt's defense will have be that he had no idea the RN would decide to let the nazis across the channel just for kicks.
Ultimately this probably leads to a Sov-wank, and conspiracy theories during the era when Soviet influence in the US is being discovered and rooted out that Roosevelt deliberately let the Nazis take England in order to clear the ground for Soviet domination of most of Europe.