I believe that you are mistaken: the political jargon should be "yea" and "nay". "Yep" is a colloquialism for "yes" although quite improper for a formal context.
Linguistically, "nay" comes from Old Norse and thus "yea" comes from Old English probably as a compliment, meaning "so be it". Both terms were embraced by Middle English.
Nowadays, "yea" and "nay" are restricted to the political arena. American English took these terms from British English during the colonialization of the New World where they fossilized while British English might have further evolved from surrounding language groups.
Eric Paul Monroe