1640s, "operation of an army in the field," during a single season, in a particular region, or in a definite enterprise; from French campagne "campaign," literally "open country," from Old French champagne "countryside, open country" (suited to military maneuvers), from Late Latin campania "level country" (source of Italian campagna, Spanish campaña, Portuguese campanha), from Latin campus "a field" (see campus).
Old armies spent winters in quarters and took to the "open field" to seek battle in summer. Generalized to "continued or sustained aggressive operations for the accomplishment of some purpose" (1790); in U.S., especially "political activity before an election, marked by organized action in influencing the voters" [DAE], attested from 1809.
"to serve in a campaign," 1701, from campaign (n.). Political sense is from 1801. Related: Campaigned; campaigning; campaigner.