Yours, Etc.: Origins and Uses of 8 Sign-Offs
1540s, "plan of action, scheme, design," from Middle French plateforme, platte fourme, literally "flat form," from Old French plat "flat" (see plateau (n.)) + forme "form" (see form (n.)). The literal sense of "raised, level surface" in English is first recorded 1550s. Political meaning, "statement of party policies," is from 1803, probably originally an image of a literal platform on which politicians gather, stand, and make their appeals, perhaps influenced by earlier sense of "set of rules governing church doctrine" (first attested 1570s). Railroad station sense is from 1838.
A political party's or candidate's written statement of principles and plans. A platform is usually developed by a committee at the party convention during a presidential campaign.