The OED’s earliest attestation of the word unelectable is a 1932 biography of the socialist George Bernard Shaw. The passage in question is about left-wing Shaw’s election as Vestryman and Borough Councillor of St Pancras:
In 1897, his friends Robert and “Lion” (Mrs.) Phillimore, then members of the St. Pancras Vestry administering the local government of quarter of a million Londoners, made a deal with their anti-Progressive opponents whereby certain nominees of theirs were elected without protest, the number of candidates being by agreement no greater than the number of vacant seats. In no other way could Shaw have secured election; for he boasts with truth that he is a hopelessly unelectable person under any existing democratic franchise.
This is succour for all of us who voted for the new Labour leader Jeremy Corbyn, Vest(ry)man and MP for neighbouring Islington North. All that needs to happen for him to become prime minister in 2020 is for there to be no other candidates.