David Long has written an excellent work on the Presidential election of 1864, which decided that the Civil War, by then going for over three years and at appalling cost in blood and treasure, should be fought to a finish rather than ending in southern independence or a compromise peace - either of which would have preserved negro slavery for decades to come.
One eye-opener in particular was the extent to which Democrats were apparently prepared to take what amounted to openly pro-Confederate stances. Thus the Democratic candidate for Governor of Illinois declared his willingness to use State militia to resist any further Draft calls. And while I always knew that Democrats in that era were decidely racist, I was taken aback by how virulent much of it was. Substitute Jews for Blacks, and some Democratic campaign speeches would have been quite at home in Der Sturmer.
Long also goes some way to endorse Lincoln's claim that McClellan, if elected, would have secured his election on such ground that he could not save the Union after taking office. He makes some good points, notably that Black troops, who were now some 20% of the Union Army, could not have been expected to fight for an administration that went back on Emancipation, while it would have been all but impossible for President McClellan to issue any further Draft calls when his party was so strongly against them. However, while correctly notting that a new Democratic Congresss would not meet until December (unless McClellan called it earlier) he seems to suppose that the old Republican one continued until then, and might have tried to impeach the new President. In fact the Old Congress would have ended om March 4, along with the Lincoln Presidency. He also ignores the probability that Lincoln might have had the lame duck Congress vote war credits and authorise Draft Calls through to December '65, so that Mac could have continued the war at least until then without needing Congress at all. In such a case, the Confederate Army would have had to fight on for at least eight months longer than it actually did, and maybe longer still. After all, if Congress reconvenes in Dec 1865 to find Sherman already marching to the sea, it is hardly going to call off the war at that point, no matter how Copperhead it is.
In short, Long has a good case, but maybe overstates it just a wee bit. Still, like I said, an excellent book and well worth a read.