republican (adj.)
1712, "belonging to a republic, of the nature of a republic, consonant to the principles of a republic," from republic + -an. The French republican calendar was in use from Nov. 26, 1793 to Dec. 31, 1805.
republican (n.)
"one who favors a republic or republican principles" (or, as Johnson puts it, "One who thinks a commonwealth without monarchy the best government"), 1690s; see from republican (adj.). With capital R-, in reference to a member of a specific U.S. political party (the Anti-Federalists) from 1782, though this was not the ancestor of the modern U.S. Republican Party, which dates from 1854.