Had a 5th century Roman emperor refused to surrender, after suffering the losses the Roman Republic did in the Second Punic war, his generals and governors would have defected and seceded, respectively. Especially once that emperor tried to press more men into service after over 100,000 had already been killed by an invading force. The Senate was able to remain in power because of a cohesion that didn't appear to be present in rival states or the Late Roman Empire.

The fractious nature of Senate politics was a consequence of a culture and set of institutions that encouraged and allowed domestic conflicts to be settled within the confines of established political processes, so that the Senate as an institution, while often beset by bitter internal political strife, did not face challenges from external centers of power within Roman society. The power and resiliency that this gave the Senate allowed it to maintain control even when Rome was under extreme stress, like it was in its depopulated state during the Second Punic War when it managed to raise two dozen legions.

>>The only groups that remained loyal to Rome during the second Punic war were the northern Latin cities.

You're correct that there was a lot of defection, but most cities in the Italian peninsula remained loyal to Rome.