Why did Lincoln agree to have a Democrat as his vice-president rather than someone from his own political party?

"august 19th (1864)- how they quarreled and wrangled among themselves- alabama and mississippi, all were loud for joe johnston, save and except the long-haired, one-armed hero, who cried at the top of his voice: "oh! you all want to be kept in trenches and to go on retreating, eh? " "oh, if we had had a leader, such as stonewall (jackson), this war would have been over long ago! what we want is a leader! " shouted a (wounded man). they were awfully smashed-up, objects of misery, wounded, maimed, diseased. i was really upset, and came home ill. this kind of thing unnerves me quite." mary chestnut, a diary from dixie this entry from the diary of mary chestnut illustrates a) the lack of medical technology possessed by the confederacy. b) the military superiority of southern troops in the civil war. c) the effects of the emancipation proclamation on the southern economy and society. d) the loss of morale and confidence by the confederacy towards the end of the civil war.