Pierce wanted to run for re-election. Many northern Democrats,however, objected to his support of the pro-slavery legislature in Kansas.

VOA: special.2009.05.07

Northern Democrats refused. The opposition Republican Party expected to gain votes from dissatisfied Democrats.

VOA: special.2009.07.09

He said Northern Democrats were tired of defending the interests of the South.

VOA: special.2009.07.09

Further, he rallied most of the northern Democrats to the Union cause.

WHITEHOUSE: Abraham Lincoln

Many African-Americans migrated to the northern industrial cities, and northern Democrats, with their eyes on electoral gains, began to woo their vote.

CNN: Obama giving some black Republicans 'heartburn'

Many northern Democrats were demoralized and there was open talk in places like Tammany Hall (the New York City Democratic Party) about the need to distance themselves from slavery.

FORBES: In Lincoln (2012 Movie), Why Did Lincoln Rush The Vote On The 13th Amendment?