What is the role of public opinion in the civil war?

By 1864, critical, because that was an Election year. Northern morale was rock-bottom because Grant was taking big casualties every day at Petersburg, while making no apparent headway, and he had ended the system of prisoner-exchange, so the prison-camps were becoming horrendously overcrowded.

If Lincoln had lost the election, the North would have lost the war - and he actually thought he was going to lose.

In the end, he was saved by some timely battlefield successes by Sherman, Sheridan and Admiral Farragut.