Final answer:
Those against slavery referenced the quote from the Declaration of Independence, asserting that "all men are created equal" and have unalienable rights to "Life, Liberty and the pursuit of Happiness," which slavery contradicted.
Step-by-step explanation:
The famous quote that those against slavery felt was not being upheld is from the Declaration of Independence, which states, "all men are created equal" and are endowed with "certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness." Abolitionists and others opposed to slavery pointed out the inherent contradiction between this statement of equality and liberty and the existence of slavery in the United States.
Figures such as James Otis, Arthur Lee, and William H. Seward highlighted the incongruity between the nation's founding principles and the brutal practice of human bondage, arguing that freedom is the birthright of all mankind and denouncing slavery as morally repugnant and contrary to the law of nature and nations.