It also allows for wordplay about "sewer rats and Democrats and all the other kids of rats".

The word Republican would allow for similar wordplay with respect to bodily functions.

Political labelling is an interesting game. I was amused by the way that at a certain stage of events in the wake of the collapse of the USSR the media took to referring to the Communists as "conservatives". (Accurate enough, of course, to anyone with a dictionary handy.)

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However I don't think the term "Democrat" can wholly be put down to Joe McCarthy. After all, how else would you refer to a member of the Democratic Party? Couldn't very well say "he/she is a Democratic".

Whereas with Republican you would. And you couldn't go referring to them as "Republics".

Language is curious and inconsistent. And usage isn't always just down to political manoeuvring.