Sebastian was a king of Portugal who died in 1578 in a war against the Arabs. His body was never found. After that, the Avis dynasty was over, and Portugal became a part of the Spanish kingdon. This situation created the myth of Sebastianism: the belief that one day Sebastian would return and save Portugal from the Spanish domination. He would be the savior.
https://en.wikipedia.org/wiki/Sebastian_of_PortugalI think that the anti-Trump republicans are looking like the "sebastianistas". They are waiting that one day the anti-Trump republican will arrive riding a horse, get the nomination and save the GOP. Republican pundits have been picking numbers and past histories in order to elaborate theories that one day Donald Trump will loose ground. And he is not loosing ground.