1712, "belonging to a republic, of the nature of a republic, consonant to the principles of a republic," from republic + -an. With capital R-, "of, pertaining to, or favoring one of the various American parties that have been called Republican," by 1806 (the modern GOP dates from 1854). The French republican calendar was in use from Nov. 26, 1793 to Dec. 31, 1805. Earlier adjectives included republical
(1650s), republicarian (1680s).
"one who favors a republican form of government or republican principles" (or, as Johnson puts it, "One who thinks a commonwealth without monarchy the best government"), 1690s; see 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. In between, National Republicans was a name of the party that opposed Jackson and rallied behind John Quincy Adams in late 1820s.