I haven't seen this movie, but I can tell you that gangs held incredible sway over all big cities on the Eastern Sea Board in the early and mid 1800's.

Ever hear about a fellow named Boss Tweed?
http://upload.wikimedia.org/wikipedia/commons/e/e2/Boss_Tweed,_Nast.jpg

He was a powerhouse politician who had full control of the city (New York) due to his voting ring, a voting system which would guarantee him and whoever he supported, power. How these voting machines made of people got started is both simple and ingenious. They capitalized upon immigrants coming fresh off the boat, mostly Irishmen and Germans in the early/mid 1800's http://www.victoriana.com/history/images/know.JPG. Both these groups were hated by the "native" anglo saxons, "oh no, these immigrants are here to take our jobs"- sound familiar to you? And were discriminated against en masse, NINA- No Irish Need Apply was pasted over every wall.

But wait, I'm Im a immigrant, how do I get work? How do I get a home? How do I feed my family? You talk to Boss Tweed of course! Or whatever ethnic boss sympathetic to your people. They give you a decent (well paying) job, a house (apartment really, even if its the 1800's New York was still New York) and whatever you need to settle your family. And just like the Godfather, who is by the way, a good analogy for this kind of relationship, they'll ask you for a favor. What favor? Your vote. Why does my vote matter? Because you immigrants are numerable enough to form a plurality over everyone else.

With this method, these bosses controlled the cities elections which meant controlling every aspect of each city. New York was famous for being run by people powered by Irish Immigrant votes. And what did these bosses do with all that power? They abused it for money of course. Drugs, prostitution, alcohol you name it.

Citation

Ackerman, K. D. (2005). Boss Tweed: The rise and fall of the corrupt pol who conceived the soul of modern New York. New York: Carroll & Graf Publishers.