We had an ocean between them and us. No one had a delivery system at that time which could have reached across the ocean.
They would have probably used what they could against the Russians, but ultimately it would have done little good. The Nazis were losing behind the lines against the partisans and all the other thousand little irregular warfare activities.
And nuking the battlefield, which is what the Nazis would have done, would have hurt them just as much as the Soviets. No one had any idea what that weapon would really do. We at least had distance between our troops and the Japanese, there was little or none between the Nazis and Soviets, and Hitler would not have cared anyway.
The act of bombing the Soviets would have galvanized the USA into hyperactive research and activity, which would have given us what we needed to defend ourselves.
“The German Army in fighting Russia is like an elephant attacking a host of ants. The elephant will kill thousands, perhaps even millions, of ants, but in the end their numbers will overcome him, and he will be eaten to the bone.”
—Colonel Bernd Von Kielst, Wehrmacht, 3rd Reich