Republican-Controlled Soldier Vote
The Democrat party was hopeful of victory in the 1864 elections as the war had reached a stalemate with Sherman checked at Kennesaw Mountain and with Grant “hurrying so many into death” before Richmond in June. Lincoln sensed that his administration would not be reelected, and perhaps didn’t realize his subordinates would throw the entire force of the War Department behind his reelection to ensure Republican power and hegemony.  Northern officers focused on career advancement and power made sure Democratic ballots were lost.
Bernhard Thuersam, Chairman
North Carolina War Between the States Sesquicentennial Commission
"The Official Website of the North Carolina WBTS Sesquicentennial"
Republican Controlled Soldier Vote:
When the [Democratic] Convention met at the Chicago Wigwam on August 29 [1864], Western delegates generally favored a peace platform and a peace candidate. Eastern men and some Westerners like Cox, wanted a war candidate. The choice fell on [George B.] McClellan, whose nomination Cox seconded. To pacify the peace element, George Pendleton was given second place on the ticket.
Early in September McClellan’s letter of acceptance emphasized the war’s original objective, restoration of the Union, but rejected peace-at-any-price doctrine. He would prosecute the war for a speedy restoration of the Union with a generous recognition of Southern rights within that Union.
[Sherman’s] capture of Atlanta gave new strength to the Republican Administration. The plans of dissident politicians to replace Lincoln were frustrated, and [John] Fremont withdrew from the [presidential] race. Anti-Lincoln radicals were appeased by the retirement of Montgomery Blair from the cabinet. The Republicans entered the final weeks of the campaign in a strong position.
Back in Columbus [Ohio, Samuel S.] Cox found that McClellan’s acceptance letter had raised some “tall cussing”….by [Clement] Vallandigham, who had returned from exile. There was a 5,000 vote Republican majority in 1863 in his district. He felt “like a lamb” being led to slaughter and considered turning down the [Democratic] congressional nomination. By September [1864] he couldn’t resist the lure of the political area and accepted the nomination, hoping that with “a fair response from my friends in the Army – I can get through.”
On October 11, 1864, the Ohio State Journal called Cox “this little dodger” and told its readers to “repudiate him forever.” For his campaign, Cox spent $2,000 of his own savings before sending his brother East with an appeal for assistance.
When he tried to rally support among soldier votes at Camp Thomas near Columbus, he was “driven three or four times” away at bayonet point.”  His ballots were not accepted when soldiers attempted to vote for him.”
(“Sunset” Cox: Irrepressible Democrat, David Lindsey, Wayne State University Press, 1959, pp. 86-87)