I don’t think that most people have any idea just how anti-war most Americans were going into WWII. Nor do I think people understand how anti-British many Americans were.
There were probably several reasons for the anti-British sentiment. We had fought Britain in two wars for national survival and had a tough time of it. People were still taught history then, and they thought of Britain as an aggressor. Globally, Britain was still a colonial power, and we were anti-colonial by birth right. Finally there were a lot of Irish immigrants that had absolutely no use for the British at all.
Even after WWI we were still making warplans to fight the British and defend our Eastern seaboard from the Royal Navy.
The Commies sure were "anti-war", but that all changed when Hitler invaded their beloved Soviet Union. Then the press' attitude towards the isolationists suddenly changed.
Another significant sore point was Britain’s use of war loans being used to reloan to other allies and deprive the United States of the jobs the war loan to Britain was supposed to produce. The Britain rubbed even more salt in the wound by defaulting on the multi-billion dollar debt when the debt when the allies they reloaned the money to defaulted on their repayments of the reloans to Britian. Britain simply wasn’t trusted to keep its agreements or honor the contracts to produce american jobs in excchange for the loans.