Hi there. I've written answers on the medieval Italian political system as well as comments on the instability of that same system which might interest you.

In short, the densely populated and urbanized heartlands of the Po basin sustained a stable population living in cities, where only with difficulty authority could be concentrated in the hands of a single ruler. Even under the Frankish Empire, imperial representatives and margraves alike had to contend with clergymen, judges, and lawmakers seated in the cities who were elected, in the Roman fashion, by colleges of prominent citizens. Indeed, thousands of large and medium sized landowners split their time between country estates and urban residences, while small farmers, aided by the shallow waterways crossing the plain and a weak legal precedent for obligations to their landlord, regularly traveled to town in order to bring their goods to market and purchase finished goods made by urban artisans, all contributing to the liveliness and bustle of urban life. Thus the governmental model which propped up these Italian cities was a complicated amalgam of very different groups: the urban clergy, the large and small landowners, the middle-class professionals, the artisans, and of course the working poor. The Italian urban communes were strange, ad-hoc polities uniting different legal jurisdictions, prerogatives, and public will, with the end result being a patchwork of decentralized local authorities governing Northern Italy.

Between the 13th and 15th centuries, communities like the Milanese, Paduans, Veronese and Florentines were almost continuously involved in some kind low-level conflict, often bullying smaller surrounding towns into submitting in order to establish a ring of influence and an area of political and economic domination. Few cities would remain republics as bordered solidified into the dozen or so states of the renaissance, however, many of those that did retain republican institutions were some of the most successful.

I've written a bit of a long-winding narration of the political history of Medieval Italy here which might also interest you.