The United States was still getting over the turmoil of World War I. The general public was not ready to join another war, opting for neutrality. A poll taken in 1939, after the outbreak of war, showed 94% as being against going to war.
At the time, it was assumed it was going to be another WWI-like stalement. Once France fell, most people were probably resigned to the fact that getting into the war was inevitable.
We simply could not risk losing the Royal Navy to the Nazis, since we diverted so much of our Atlantic fleet to the Pacific to face the threat from Japan.
stalement = stalemate
How exactly was the Royal Navy going to be lost to the Nazis?