Phst, why do I even try to argue with you on points about Latin History Quatro?
Actually, because of the methodology of elections, one of Rome's fears may have been defection, but it wasn't one of their chief ones. The process of election was the courting of favor, through giving gifts/paying for celebrations within the Roman Republic. As a general whole, only the heads of individualized city-states considered the idea of breaking from the Republic, as most citizens realized the idea of being under Carthagenian rule was worse than the idea of being under Roman.
Eh, I don't really think it was because he wasn't well equipped enough. (Actually it's a subject of debate among scholars to this day.) It seems more likely that he was attempting to rally Roman areas to his banner, and failed. (As he tromped throughout Italy, and begin trying to form his own military command units, not to mention set up supply depots for any type of future war effort on his part.)
As for the manpower, they couldn't really call up unlimited reserves, as they were relatively confined. However, once they did lure away Hannibal through "foreign" (or out of Italy) military action, the units he had attempted to form for command/recruitment purposes, turned out to actually support Rome, which gave them both an already formed fighting force, as well as people trained in the art of Hannibal's tactics. This was evident when the seperation, near the modern day Vatican, gave Rome nearly a hundred thousand soldiers with an already prevailant command structure.
While you are correct that it was the invasion of Spain that lead to the possibility to attack Carthage, which was through naval routes, not land routes, actually. It was the defeat of Carthage's Navy outside the reefs of Siciliy that allowed for an actual Roman invasion.
Although I do have to agree with you, Hannibal's victories were the major concern for the Republic, as they feared that should Rome fall, the rest of their fledgling empire would begin to crumble without any hope of a leadership base from the home city.