That wasn’t the key difference. Although it could be said that one of the probable reasons Lincoln postponed the nomination was to not further divide the Union. Trump’s nomination will cause further division.
Lincoln had already appointed four Supreme Court Justices. That included a Justice to fill a new 10th seat that was added to correspond to the newly created 10th circuit established for California and Oregon. That was a bit of court packing, but Lincoln filled the seat with a Unionist Democrat.
Justice Taney died October 12, 1864. Taney was not anti-slavery (he wrote the majority opinion in the Dred Scott case.) That gave Lincoln a solidly pro-union and anti-slavery court. He had no immediate need to fill that seat.
Lincoln was running against George McClellan, his former general-in-chief. Lincoln’s bitter rival was the Confederacy, not McClellan, although McClellan was against the abolition of slavery. But Lincoln’s prime concern was the preservation of the Union, even it meant losing the battle on slavery.
Months earlier when it became clear that McClellan would be the Democratic nominee and that he might win, Lincoln wrote a secret memorandum to be revealed if it became clear that McClellan would win. In that memo Lincoln pledged that if McClellan won, he would put all his effort to work with McClellan to use his majority influence and his own Presidential powers to end the war before McClellan was inaugurated. It appears Lincoln did not think McClellan would or could preserve the Union on his own.
Lincoln never said exactly why he didn’t nominate a Justice before the election. But the key factor was probably because he did not want to further divide the Union when he was already struggling to with a divided country. But the other likely factors are that he had no immediate need to fill the seat, putting in an anti-slavery Justice could lose him votes among anti-war/pro-slavery democrats, he was likely to win anyway, he wanted to bring together the Union, he wanted to work with McClellan if he lost to at least defeat the Confederacy and preserve the Union, and the matter of slavery would likely be resolved by Constitutional amendment if the Confederacy was defeated.
