Who was the Democratic Party presidential candidate in the 1864 election?

This election occurred during the US Civil War.

Leading up to the political conventions, with the US losing several battles to the Confederates, there were many different political factions, all trying running on different ways the war could end (some wanted the war to end immediately and allow the Confederacy to be its own country, some wanted a negotiated peace that would allow the US to be whole again but allow slavery, some wanted to continue using force to end the Confederacy).

Abraham Lincoln was the incumbent and ran for re-election (which he did win - and was assassinated roughly one month after he was sworn into office). He was the Republican candidate (that had renamed itself the National Union party so that Democrats could vote Republican without having to literally vote Republican - they could vote National Union).

McClellan was the Democratic party nominee. The democrats were split between those who were pro-war (such as McClellan, and were known as War Democrats) and those who were against (known as Peace Democrats). The decision was made to make a War Democrat the Presidential candidate with a Peace Democrat as the Vice Presidential candidate).

After the two candidates were selected, the Union won several important battles, meaning that the country was more willing to keep the war going and try to defeat the Confederates.

https://en.wikipedia.org/wiki/United_States_presidential_election,_1864