Can you imagine the outrage if this happened today?
And did you check out the republicans wanting to abolish the poll tax by constitutional amendment?
Times change....
The Holocaust is unique in history for it's state-organized, systematic, industrialized mass murder.
But there have been a large number of mass-murderers since WWII, including Mao Tse Tung in China, Pol Pot in Cambodia, Saddam Hussein in Iraq, and any number in Africa, none of which have excited much attention in the US, at the time.
For example, Saddam's mass murders only became a real issue here when the Bushes needed them to help justify their own military adventures.
Point is, so far as I can remember, mass murders of alien populations were never, by themselves, a casus belli for Americans.