I didn’t know the history behind the pejorative usage. Thanks for pointing out.

EDIT: I'm a recentish immigrant, so help me out with this inconsistency: Why Democrats and not Democratics? Why Republicans and not Republics?

Democrat is a noun, Democratic is an adjective. Republican is both (depending on context).

"Republican" and "Democratic" are modifying the noun "Party" when referring to the political organizations, so you use the adjective in those cases.

Thanks!