It's because of political bosses like Mike Scully that Chicago politics still have a reputation for corruption. (For more on the fascinating real-life history of Chicago's corrupt politicking, check out this link.) Mike Scully is the richest man in Packingtown. He has iron-fisted control over the elections in the poorer parts of Chicago. He is a Democrat, and he arranges Democratic victories by buying votes, encouraging people to vote multiple times for money, and generally rigging the whole election process. Mike Scully also runs the dump where Juozapas scrounges food. His corrupt selling of elections allows business interests to thrive in Chicago, against the real interests of the voters. Because of Scully's influence, lots of expensive city hygiene and planning never happen. The deep potholes in the roads are Mike Scully's fault, which means Mike Scully is indirectly to blame for Baby Antanas's death.
Jurgis briefly works for Mike Scully as his union contact when Scully is trying to arrange an election victory for the Republicans in exchange for a future Democratic triumph. Scully's willingness to wheel and deal with his opponents proves that the difference between "Republican" and "Democrat" is mainly window-dressing in the world of The Jungle. It doesn't matter what either party stands for as long as both parties are totally in the pockets of wealthy businessmen. Jurgis's direct experience with the corrupt election process is one reason why he is so intense about demanding Socialist Party success at the end of the novel. The electoral system is so broken that the only way to guarantee change is by voting for a third party.