My understanding is that most people after the war claimed they'd never heard of the Holocaust during the war.
Indeed, this is a key element in Holocaust deniers' conspiracy theories -- they claim the Holocaust was invented after the war by those dastardly CommunistJews, and it never really happened.
But these articles in the New York Times prove that at least some Americans were fully aware of the Holocaust during the war.
So the question has long been: why didn't more people know about it?
The link (post #19) provided us by fso301 supplies a key element -- Arthur "Punch" Sultzberger.
Drowned out as background noise by the events of a titanic global conflict. People "knew," but given everything else, it wasn't "real" until GIs walked through Buchenwald.
The Gulag was never "real." GIs never walked through the camps at Magadan.
And there was the Rabbis’ March on Washington in October 1943:
http://www.youtube.com/watch?v=pKQNHNdOmow
https://www.jewishvirtuallibrary.org/jsource/Holocaust/march.html
In those days the Democrat party was a coalition between Southerners and northern immigrant populations — groups which didn't always agree with each other, and which wily politicians like FDR, would try to reconcile by, in this example, slipping out the White House's back door to avoid embarrassment.
Then as now, many non-religious Jews (i.e. “Punch” Sultzberger) were loyal Democrats first, and Jews second (or third, if at all) in their political orientations.
Today, many observant Jews (Mark Levin to cite one) are similar to practicing Christians in their politics — conservative, Republican, pro-Isreal.
And today's versions of old Sultzberger, like lemmings, blindly follow Democrat party lines, even to the detriment of Israel.