First, IMO it would be possible for the US to declare war on Germany without the Pacific War having already started. A late 1941 Gallup poll showed that American opinion was divided as follows:
20% - no opinion
20% - isolationist
20% - interventionist
40% - support the Allies as necessary even at the risk of war
At that time, Gallup often reported a secondary result, of only people important enough to be listed in Who's Who. The secondary result was 55% interventionist.
The problem is, I don't know what would tip the 40% group to outright interventionism. There had already been incidents in the Atlantic; I can't see how a few more would be decisive.
But assume that the country follows the elite as measured by Gallup, and that the US declares war on Germany in July 1942, and also that Japan for whatever reason never attacks any of the Allies.
The latter has short term consequences: Britain is not distracted from the North African campaign, pre-empting Rommel's early 1942 comeback. It is distinctly possible that Britain finishes the North African campaign by mid-1942, which has consequences for French North Africa. Mainly that the pro-Allied element there will be strengthened, and the US/UK can move in a lot easier.
There are also consequences for the USSR. If Japan does not commit to attacking in SE Asia (and the US), Sorge cannot report that to Soviet intelligence, and the potential threat to the USSR from Japan would pin down more Soviet troops in the Far East. Also, if as noted the North Africa campaign ends a year early, the German forces sent there in late 1942 would be retained and be available for the Eastern Front. Unless the US and UK can gin up some sort of attack into continental Europe.
Suppose Libya is done by June, and French North Africa flips to the Allies in August... If OTL the US/UK could stage TORCH in November 1942, ITTL they can do something comparable or even larger. They would have the support of French North Africa, and all the resources sent to the Pacific OTL.
(A bit of weirdness comes up here. What are the US Marines doing? There's no fighting in the Pacific, and the Army will be dead set against any USMC participation in the ETO. They'll be last on the list to get any amphibious craft or airplanes. Will the Marines basically sit out the war? IIRC, after the Marines got most of the headlines for the battle of Belleau Wood in June 1918, resentful Army brass made sure they never saw action again.)
With Stalingrad roaring, the Allies have to attack somewhere. Where? France is out, I think. Sicily would be very ambitious, Crete out of air support range. A massive raid on France seems most likely.