By February 1812, the position of the French in Spain was deteriorating especially in the north. The failure of the 2nd invasion of Portugal, combined with the unsuccessful ongoing siege off Cadiz demonstrated the decline of French power from its height in 1809. The fall of Ciudad Rodrigo in January of that year opened northern Spain to an invasion from Portugal, and set the stage for Salamanca, and the series of disasters that would see the French driven from Castile back to Catalonia within a year.
However, this was not the first time that French fortunes in Spain had fallen. In June of 1808, the British and their Spanish allies had nearly driven the French from Portugal and Spain, and had advanced to the foot of the Pyrenees. It was in those circumstances that Napoleon had campaigned in person in 1808 to revive the fortunes of his empire in Spain. He largely succeeded; while the British Expeditionary force was not entirely eliminated, it was forced to evacuate, and nearly every major Spanish city occupied by the French, and were nominally subjected to the rule of Joseph Bonaparte.
It was in these circumstances that Napoleon started to prepare for his invasion of Russia, an enterprise that diverted over 30,000 troops from his army of Spain, setting the stage for the precipitous decline of the French position in the Iberian Peninsula.
What if, instead of deciding upon an all-or nothing campaign in Russia, Napoleon had opted for another campaign in Spain in 1812, using most of the same multi-national forces that he would have otherwise employed in Russia? Could he have repeated his success in 1808, and perhaps followed up with a third invasion of Portugal? How long would Napoleon have to campaign in Spain before another coalition was formed against him by his erstwhile allies, especially his father-in-law Francis? How might this have affected the long-term prospects of the French Empire?