One aspect worth considering is that northern Italy - in particular places like the Aosta Valley, Emilia-Romagna, Umbria, and Marche - were some of the strongest centers of support for the PCI and PSIUP. These also happen to be some of the strongest centers of support for the Republican option in the Institutional Referendum.

Data for both elections can be found in Elections in Europe: A Data Handbook by Nohlen and Stover. Also, A History of Contemporary Italy: Society and Politics, 1943-1988 by Paul Ginsborg is a good general read on the development of Italian politics and society in the aftermath of WWII.

Hi there! I answered a very similar question here that you might be interested in.

The German-occupied pupped state propped up in the north after the King's armistice with the allies in 1943 had a short and unhappy life, beset by unrest and civil strife. The south, on the other hand, lived through a much milder occupation by the allies. I wrote about this dynamic in this short answer, with links to longer answers on more specific topics. Further, King's decision to leave Rome and seek refuge behind the allied lines in Southern Italy had left many in north feeling angry and abandoned.