You may want to check your premises - there is little evidence to suggest that Lincoln was exceptionally popular as President, and indeed a fair case to be made for his unpopularity. While polling data is obviously unavailable, the Republican Party lost hugely in the midterm following his election. His re-election could easily be credited with the fact that Lincoln was not McClellan, who ran on a platform of ending the war at any price just at the time that the war started going well.

Lincoln' reelection in 1864 completely hinged on the Union gaining a decisive victory. He had written a letter for the next president, completely thinking he was going to be beaten. Antietam was the first real turning point that rallied the North to back Lincoln's reelection. That and the ballot boxes being sent into the field for soldiers to vote. Lincoln received 80% of the soldier vote.