in the United States, is the policy of a political or religious party. Of course the meaning is the policy on which the party stands. An American revival. Each separate principle is a plank of the platform. Queen Elizabeth, in answer to the Supplication of the Puritants (offered to the Parliament in 1506), said she “had examined the platform, and account it most prejudicial to the religion established, to her crown, her government, and her subjects.”
Again, the Rev. John Norris writes, in 1687, that Plato said, “God created implying that all things were formed according to His special platforms, meaning the ideas formed in the divine mind.” The word has been resuscitated in North America. Lily, in 1581, says he “discovered the whole platform of the conspiracie.” (Discovery of the New World, p. 115.)
“Their declaration of principles- their `platform,' to use the appropriate term- was settled and published to the world. Its distinctive elements, or `planks,' are financial.” —The Times.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894