I think if the US did not declare war in 1942, they would have soon afterwards over some other cassus belli. America was already at war with Germany (after Hitler's dastardly and unprovoked declaration of war on December 27th 1941--a day that will live in infamy in FDR's words). They were literally just begging the Japanese to shoot, heck, the SS President Adams was in Japanese territorial waters.
So, any sort of delay might make it where Japan wins the war in the East. Operation Ichi Go pretty much cowed the Chinese and the Indian revolt made it look like that Japan was going to knock the British out of the war in Asia. All of those technology transfers from Nazi Germany to Japan were starting to pay dividends by the end of 1945. Remember how the Enola Gay was shot down in March 1946 over Nagasaki by a Japanese Nakajima Ki-201? Sure, we got a Nuke in here or there, but most of Japan's industrial capacity was in the countryside, Korea, and Manchuria to begin with. The war was a pretty close thing as is if it were not for the fact that the blockade was starving them all to death. Delay the way a few months and Japan wins in Persia, the USSR is defeated in Manchuria (they were close to defeat anyway when the Japanese did make their formal surrender) and Japan pretty much has no more fronts other than the home front. Once enough Ki-201s hit the skies the atomic threat may be neutralized.