It was a landslide, but that didn't always seem likely.
Perhaps the best POD to make this happen would be the Union losing badly at Gettysburg. Then Lee turns East, conducting cavalry raids as far North as Harrisburg, taking York, and then heading South to threaten Baltimore. Lee is eventually forced to withdraw, but takes Frederick, MD en route to VA. The whole campaign causes a lot of damage and shakes confidence in PA, MD, and DC. Confidence in Lincoln is also further shaken.
Radical Republicans nominate John Fremont for President.
No Democrats join the National Union Party effort and several contest Lincoln for the nomination.
Lincoln ultimately wins the Republican nomination, but Fremont stays in the race as an independent and splits some of the Republican vote.
McClellan is able to demonstrate that Republican efforts have led to the secession, that Republican efforts in the war have failed, and that they can't even protect the great cities of the US.
The military doesn't allow most troops in the field to vote and those who do are very split.
McClellan wins a narrow victory.