What if FDR died of ill health in mid-1943, early June, leaving Henry Wallace as the US president?
He was known to be pretty far left even by the standards of the times and had gotten into a number of feuds with other government officials, while not being popular with the party.
Wallace was not actually particularly leftist. He was a businessman who had been a Republican until FDR appointed him Secretary of Agriculture. A "Progressive" Republican, to be sure, but no Red. But like nearly everyone in the West, he had no real comprehension of what the USSR really was. In 1945-1948 he was taken in by Soviet "peace" propaganda (as echoed through agents of influence and useful idiots in the West), and was maneuvered into the "Progressive" Presidential run. Within a few years he realized he'd been played, and became hard-line anti-Communist.
However, in 1948, like most Westerners, he was bemused by the Soviet pose of being just a little more liberal, and of course valiant foes of Nazi Germany - even more than FDR and his staff.
But the real problem with Wallace was his loose lips. Wallace's sister Mary was married to the Swiss ambassador to the United States. Wallace told his brother-in-law a lot of things that he should not have. The ambassador was a good guy, strongly pro-American, but he reported what he learned to his government, as was his duty. The information went to Switzerland in diplomatic pouches, enciphered, and the Swiss officials who read it did not leak. But the Germans had an agent in the mail room of the Swiss Foreign Ministry, who got them copies of the cipher messages, and their cryptanalysts had broken the Swiss diplomatic cipher.
So Wallace was a leak of highly confidential information to German intelligence. IIRC, the Swiss operation was run by the
Sicherheitsdienst (SS intelligence), controlled by Walter Schellenberg.
Fortunately, two factors prevented this leak from causing any great harm. First, FDR did not keep Wallace posted on the Big Secrets of the war, such as ULTRA and the Manhattan Project. This prevented any really hot material reaching the Germans this way. Second, when Schellenberg presented his results, his rivals in the Nazi state talked down the significance of what he had gotten, and he failed to impress Hitler.
But ITTL... Wallace would necessarily be briefed on everything. He might now limit his blabbing - or might not.