"Bush" Harper is Mike Scully's second-in-command at the stockyards. He is key to organizing Mike Scully's elections. "Bush" Harper is the guy who first brings Jurgis in as Mike Scully's union plant. When Mike Scully arranges to throw this current election to the Republicans so that he can guarantee a future Democrat win, he needs a union guy who is willing to stump for the Republican candidate. Chicago unions were traditionally Democrat, so it is a tough sell.
By this time, though, Jurgis is only interested in making the system work for him. He has no morals left, so he starts campaigning eagerly for a candidate he doesn't even know. Jurgis's willingness to work hard for "Bush" Harper, and by extension, for Mike Scully, makes him a lot of money and gives him influence in the stockyards. When Jurgis goes up against Phil Connor a second time, though, even "Bush" Harper can't get him off. All Harper can do is arrange for Jurgis to pay his bail money and then skip town. Jurgis leaves the world of the Chicago political machine just as broke and isolated as he came into it.