As cities grew due to industrial expansion, hundreds of thousands of people, many of whom were immigrants, moved to the cities in search of employment. Many of them wound up working in unskilled jobs for very low wages. They lived in cramped, overcrowded tenement buildings in largely ethnic communities rife with crime, disease, and lack of sanitation and paved roads. Political machines capitalized on this situation, delivering much needed public works, sanitation, city jobs, and other services in return for guaranteed votes.
This situation was ripe for corruption. Tammany Hall, the infamous Democratic political machine in New York, used the loyalty of immigrant communities to exercise almost complete dominance over the city's politics. Their leaders, most famously Boss Tweed, used his power to siphon millions of dollars off of the many building projects the city undertook, and rigged elections, using loyal ward bosses to deliver votes from the communities they used as their political bases.
When considering the legacy of the political machines, then, historians must weigh the vital services the machines provided to their desperate constituents along with the rampant political corruption, including spectacular instances of fraud, that the machines presided over. In any case, political machines were run by hard-nosed politicians who were quicker than anyone else to grasp the new political realities caused by industrialization and urbanization.