I know a lot of people think this, but I'm not convinced. For one thing, I've run a simulation on the battle of Kursk without American bombing sucking some 30% of the German war effort away. If you apply 30% more air power alone to Kursk, the Nazis win. If you exponentially put all the 30% military anti-bombing effort in air power alone, or in tanks alone, Germany probably wins the Russian front. Moreover, with each three Jeeps you take away from the Russkies, you have to subtract one tank or two artillery pieces. They had limitations on what they could make. And remember, we supplied not only the ships that got them there, but kept the shipping lanes open in the first place, which itself would have required a large Soviet Navy. It's a ripple effect, and all of this presumes that without the U.S., Hitler would still have lost the Mid-east (he wouldn't) or that the Germans and Japanese wouldn't have linked up in, say, Iran. Now, that would be a problem.
I'd say it's 50/50 if Russia still wins without us in there---and a longer war very well could have led to an internal revolt vs. Stalin.
f you apply 30% more air power alone to Kursk, the Nazis win. Kursk was in 1943. One way to look at the Russo-German war is to think of 1942 as a wash. The Germans ended up roughly where they started. Even if the Germans had won at Kursk, where would they have gone? They surely didn't have the ability to get to the Volga again.
In 1942, the whole western theater was a side show for the Germans. The American and British contribution in 1943 did allow the Soviets to really start kicking the Krauts butt. That said, the war would have dragged on for many more years without the Allies pushing from the west.