The US at the time was almost as polarized and gridlocked as it is now. The "progressive intelligensia", mostly clustered near the coasts, were paying attention to what was going on in Europe and the far East, and shuddering in their shoes, while the "conservative heartland", comfortable in their isolation, and aghast at the heresies of the New Deal, were leery of "foreign entanglements" and committed to "America First". It took Nanking in ruins, and then London in flames (London, epicenter of the English-speaking world!) to hit Americans at a gut level and start to elevate "America First" from a national to a global ambition, which resonates to this day.
Without the images of St Paul's against a flaming sky, Roosevelt could never have gotten away with Lend-Lease or swapping destroyers for bases, or escorting convoys, or most importantly, "a hundred thousand planes next year".
Cheers,
Wes