That is an overstatement. The Republicans wanted to stop the expansion of slavery into the territories and limit the expansion of the slave plantation system. The Republicans were not abolitionist before the Civil War.
The early Republican slogan was: "Free soil, Free labor, Free men," indicating opposition to slavery and the plantation system.
Related...
Unless yer awful pc or cannot handle the truth at all, you might want to read Alan Stang’s piece on Red From The Start...
“The Democratic Party fought them fiercely and tried to maintain slavery.
Why is this fact so rarely mentioned?”
I know the answer, teacher. Because the Democrats hi-jacked the narrative. Everybody believes the newspapers and nightly news.