Relistening to my Shelby Foote audiobooks, I realised he had, in fact, covered this surprising fact, but it suffered in my memory for being presented amidst the guns and guts narrative of the Overland Campaign.
Recognizing the importance of the War Democrats, the Republican Party changed its name for the national ticket in the 1864 presidential election, held during the Civil War. The National Union Party nominated the incumbent president and “former” Republican Lincoln for president and “former” War Democrat Andrew Johnson for vice president. As a result many War Democrats could support Lincoln’s Civil War policies, while avoiding the “Republican” ticket. While a large number of Republican dissidents had maintained an entity separate from the National Union party leading up to the 1864 election, they withdrew their ticket for fear that splitting the vote would allow the Copperhead Democrats and their “peace at all costs” ticket to possibly win the election.