That would have been quite a psychological blow to the Russians in 1943...and the Nazis would have been in absolute ecstasy at the news! However, I think the Russians would have soon recovered from their initial shock and would still have won the war handily in any case.

Hitler was briefly elated when the news of FDR's death came in 1945 (very shortly before the fall of Berlin.) He seemed to think it was the hand of God that had struck down the American president and that the tide would turn and Germany would somehow win the war now that Roosevelt was gone. He was dead wrong about that...

He didn't understand how the American government works, obviously. It didn't stop the USA one bit. Stalin's death wouldn't have stopped the Russians either in '43, but it might have improved their government some.