On Aug. 31, 1864, the sweltering galleries of Chicago's largest assembly hall, known as the "Wigwam," erupted with wild yells as Gen. George B. McClellan was nominated as the Democratic Party's candidate for president. The delegates had reason to be exuberant. The North was sick of war, support for Abraham Lincoln was plummeting, and they had an attractive candidate. But if McClellan had won, it would have been the last election in United States as a unified nation.

Handsome and self-confident, the 37-year-old McClellan...