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).
word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.
<a href="https://www.etymonline.com/word/republicanism">Etymology of republicanism by etymonline</a>
Harper, D. (n.d.). Etymology of republicanism. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/republicanism