I mostly saw it referred to as a saying, cliché, and even a [maxim]:
A voter who casts his ballot for a given candidate or proposal is normally probably disappointed when the persons or programs he endorses are not supported by enough other voters to become effective in the life of the community. In America, we have established the tradition of the 'good loser,' i.e., the defeated groups accommodate themselves with as good grace as they can to the victory of their opponents. Additional comfort is derived from such maxims as "Everydog has his day" and "Lose a battle to win a war."
~ George W. Hartmann (1941) Teachers College, Columbia University
First published in Psychological Review, 48