Answer:
It went against the ideals of freedom on which the nation was founded.
Step-by-step explanation:
The Declaration of Independence (1776) famously asserted: "We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights," and "that to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed." For the principle embedded in the Declaration of Independence to ring true in America, those words need to be applied to all humanity -- including non-white and non-male persons. For the government to derive its powers justly from the consent of the governed, African Americans needed to be counted as citizens and included as voters.
The Republican Party platform in 1860 pointed out the paradox between America's ideals and the continuation of slavery. The Republicans, whose presidential candidate was Abraham Lincoln, rejected the idea that the Constitution allowed for the extension of slavery into any or all of the territories of the United States as "a dangerous political heresy." The party platform went on to say that "the normal condition of all the territory of the United States is that of freedom ... We deny the authority of Congress, of a territorial legislature, or of any individuals, to give legal existence to slavery in any territory of the United States."