The passage illustrates that even if someone doesn’t know the exact position of the candidate about some subject, the person can have an idea because of their political party.
As a Republican a candidate defends conservatism - free market, free enterprise, business, strong national defense, deregulation, and other subjects, on the other hand, a Democrat probably will defend modern liberalism - social and economic equality, the welfare state, support for labor unions, social programs, and others.