In 1940, Roosevelt was looking at an election year, and, as you mentioned, he was combating an isolationist movement--politically, he couldn't help without risking his presidency. Churchill had previously asked to borrow fifty outdated destroyers, just to bolster their numbers (as destroyers were actually more useful versatile against the Luftwaffe everything than, say, a battleship), but Roosevelt couldn't even approach Congress about this without facing a storm of flak. It was commonly accepted that the British were going to be at a disadvantage throughout the Battle of Britain (I believe the odds were told to Churchill as 1-3 in favor of Nazi Germany), and their greatest bargaining chip in any peace negotiation (that which would spare Britain from losing their sovereignty) was their navy. Even though Churchill wasn't going to let that happen without a fight, Roosevelt would have felt safer knowing that, were Britain to fall, the Nazi Germans didn't have the bulk of the British Navy at their disposal. Roosevelt's last hope was that, were Britain to actually fall, the British colonies/Commonwealths and the United States would be able to form some conglomeration of freedom against the oppressive regimes of fascism and communism.

Source: Clayton and Craig: Finest Hour: The Battle of Britain (I've never actually been able to post due to not having a source, but we just read this for a World War II history class; if I need to change something about it, let me know.)

EDIT: Destroyers were actually relatively weak to air targets. They were just more versatile than a battleship or cruiser and good at hunting U-boats.