Why is Jurgis listed as a Democart? What causes him to become a Republican?

Jurgis has been involved in party politics and the political machine of the Democrats. He's also backed the unions.

Scully needs working men to make sure the Republican candidate will win (he's backing the Republican on the basis they will not run a candidate in the next election). Jurgis is told that he will get a job in the packinghouses where it will be his job to round up votes for the Republican candidate. He will be paid a weekly salary for this position. On Election Day, Jurgis goes to work. He gets many hundreds of dollars to pay for votes. He pockets twenty-five of every hundred for himself. He works hard, and when the election is over, the Republican candidate, “Scotty” Doyle wins.