Educate yourself:
The Republican Party platform of 1856 read,
“That, with our Republican fathers, we hold it to be a self-evident truth, that all men are endowed with the inalienable right to life, liberty, and the pursuit of happiness, and that the primary object and ulterior design of our Federal Government were to secure these rights to all persons under its exclusive jurisdiction; that, as our Republican fathers, when they had abolished Slavery in all our National Territory, ordained that no person shall be deprived of life, liberty, or property, without due process of law, it becomes our duty to maintain this provision of the Constitution against all attempts to violate it for the purpose of establishing Slavery in the Territories of the United States by positive legislation, prohibiting its existence or extension therein. That we deny the authority of Congress, of a Territorial Legislation, of any individual, or association of individuals, to give legal existence to Slavery in any Territory of the United States, while the present Constitution shall be maintained.” (http://www.presidency.ucsb.edu/ws/?pid=29619)
In 1860:
7. That the new dogma that the Constitution, of its own force, carries slavery into any or all of the territories of the United States, is a dangerous political heresy, at variance with the explicit provisions of that instrument itself, with contemporaneous exposition, and with legislative and judicial precedent; is revolutionary in its tendency, and subversive of the peace and harmony of the country.
8. That the normal condition of all the territory of the United States is that of freedom: That, as our Republican fathers, when they had abolished slavery in all our national territory, ordained that “no persons should be deprived of life, liberty or property without due process of law,” it becomes our duty, by legislation, whenever such legislation is necessary, to maintain this provision of the Constitution against all attempts to violate it; and we deny the authority of Congress, of a territorial legislature, or of any individuals, to givelegal existence to slavery in any territory of the United States.
9. That we brand the recent reopening of the African slave trade, under the cover of our national flag, aided by perversions of judicial power, as a crime against humanity and a burning shame to our country and age; and we call upon Congress to take prompt and efficient measures for the total and final suppression of that execrable traffic... (http://www.presidency.ucsb.edu/ws/index.php?pid=29620)