The war did polarize people into holding stronger opinions than they did before though.
Even if they started as unionists more than anything else, being opposed to the South turned into also opposing what they stood for. As evidenced by a lot of the most popular northern camp songs, matches and letters, it didn't take long for "hang Jeff davis, the traitorous scoundrel" to turn into "hang Jeff Davis, the traitorous, slaving scoundrel. Let's shoot rebels in the name of freedom!".

Wanting to shoot confederates is a weird reason to become pro emancipation, but I'll take it.