While the internal politics of the Empire (as well as any other faction divided into sub-factions) are utterly stupid in Warhammer: Total War, I still find the basic premise of the Empire campaign entirely plausible. Let's say Karl Franz won against Boris Todbringer by the narrowest margin, 8-7. Boris probably got six of the Elector Counts (mainly himself and the northern ones) and Ar-Ulric. That'd leave Karl Franz with just four Elector Counts, the three Sigmarite votes, and the Halfling Elector. So Boris might think for himself, eh, we're six provinces against four, we have more armies, so Karl Franz can get stuffed. Chances are that Ar-Ulric whispered in his ear (or, given the nature of the Cult of Ulric, shouted and pounded his fist maybe) about how he should rule, how he would have won if not for the iniquity of the Sigmarites having three votes against the Ulrican one.