The young Isaac Asimov. Not in print as far as I know but he was old enough to debate the question with his father and wrote it all down later in his autobiography In Memory Yet Green. Of course it has been something like a generation since I read this, and I may be misremembering. But he was confident, or at least hopeful, that the sheer size of the USSR would make Hitler's dream impossible as long as they chose to keep fighting. He was mainly just being a fanboy, but surely he looked at the map, looked up some statistics, and these reinforced his hopes.
Of course he doesn't count here because no one but his family was listening to him and his father was much more pessimistic.
It may not be known to everyone that the Asimovs came from a village in Belarus, and emigrated after Isaac was born, in the early '20s. Thus the father was someone who had grown up a Jew in Tsarist Russia, witnessed the debacle of Tsarist defeat, and may have had a more wry view of the worth of any Russian based society than his son who had perhaps an over-romanticized view of the Soviet Union's potentials. At least they never had to suffer the severity of Stalinist times, worse yet what the Germans did to the region they probably would have stayed in had they not emigrated. Isaac Asimov expressed a pretty glum estimate of the chances of his family's survival had they not gone to America.
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Edit--by 1941, Isaac would have been high school at least, and possibly already taking college level classes before being admitted to Columbia--he was also a published writer, and I believe even before being published hung out with groups such as "the Futurians," who included others of a pretty left wing bent, such as Fredrick Pohl. I wonder what discussions of the Soviet Union's prospects for survival were among the Futurians in general--they included young lefty street kids like Asimov and Pohl, but also much older figures of less ethnic background; it would be interesting to know what people L Sprauge DeCamp and Robert Heinlein (who was in NYC after the war started, working in a Naval lab IIRC) thought about it.
Also there are two relevant eras--before the USA joined, when despite significant resources diverted to the USSR by Britain, the Soviets were clearly going to have to win mostly on their own efforts, and after Pearl Harbor at which point all restraints on US Lend Lease aid were torn off.
All but practical ones. It is my understanding that despite Roosevelt making aid to the Soviets a very high priority, at some cost to the early US efforts, and against considerable domestic opposition, it wasn't possible for aid to flow in really massive amounts at first; the level of aid rose roughly linearly until shortly before V-E day when it was suddenly slashed back. Thus, the clear turning points such as Stalingrad were achieved before the large majority of such shipments had arrived, and the Russians did in fact largely stop Hitler entirely on their own only moderately aided power. This is why I have some confidence that even if they fought completely alone with no allies, and no other war fronts distracting Hitler, they still had a good chance of stopping the Germans and then after that point they would gain the upper hand slowly and arm wrestle them back.
But I am less dogmatic about that certainty than I used to be, and anyway it is all hindsight.
Even Churchill probably deserves some credit for betting on the hunch that the Russians would prevail despite the sober negative opinions of his staff.
Anyway note that FDR's speech is after the USA enters and the Russians have at least the moral support of knowing that they have some help on a scale that might save them even if their own efforts alone would not. Others who predicted it before the USA was drawn in should get extra credit, even if they lose points for hedging with it being a mere hope, not a certainty. (Or fear, in the case of the German generals).