If Japan and Germany had invaded in the summer or spring the SU would have been majorly fucked. Hell, if Japan simply hadn't attacked Pearl Harbor things would have been different as Republicans didn't want anything to do with Hitler. Though conquering America probably would have been a pipe dream, Germany gaining control over all of Europe and becoming so entrenched that nothing America could do would be sufficient to remove them by the time we woke up was a very distinct possibility.
Now that is something completely different to consider. Had Japan not attacked the US and Germany not been delayed in attacking the SU allowing them to take Moscow and eventually all of Europe and SU would the US have even pursued the Manhattan project when it did? If Germany would have been able to attack US before we were in serious "war mode" then all bets are off. I'm not sure how they would have controlled the skies since they weren't big on aircraft carriers and even after being crippled in Pearl Harbor we handled the Japanese Navy but it's still a very interesting thought game.