When hell freezes over. A landslide is simply not possible against a candidate like Obama. Certainly, it's possible for a Republican to win (if they are able to run as a moderate and they have charisma), but it's simply impossible to do that well against Obama due to his personality and ground game.
Herbert Hoover ring a bell?
Herbert Hoover certainly wasn't charismatic, the conditions were far worse than now, and he was beaten by someone with incredible appeal to the masses. Hoover always came across as cold and uncaring. And his win was inflated in 1928 thanks to bigots voting against the urban wet Catholic Al Smith.
