But America was turning out a phenomenal amount of war equipment by then and we still had additional capacity to build ships, planes and other equipment at a faster rate if we needed to. We were basically supplying Great Britain as well as ourselves and if England fell, we would have even more output at our disposal.
Also, if the Germans staged a successful invasion of England, Churchill would have moved the mighty British Navy to American shores and as many fighting men and materials that they could take with them. He had planned for that contingency and mentions it several times in his memoirs.
I just don't think there is any realistic scenario in which Hitler could have successfully invaded, or otherwise defeated, the United States.
How about if instead of invading the Soviet Union he became military allies with Stalin? He started out that way with Poland.