Zeitz gives some examples of such expressions from white opponents of slavery and its expansion:
In 1820, as in the revolutionary period, the belief that African Americans were human beings, as well as the accompanying disbelief that humans could be ordered hierarchically, was fundamental to the position that slavery violated both natural law and God's rule of scripture. Over fifty years before, James Otis had quoted Locke in affirming that '"creatures of the same species and rank ... should also be equal one among another."' ...This kind of thinking was very much part of the American revolutionary heritage with philosophical, religious and political aspects. Not every white citizen shared it, obviously. And in many cases the emancipatory impulse in such thinking was clearly in conflict with other attitudes. A white person could declare their faith that there are "no inherent distinctions between men" while simultaneously seriously restricting their idea of how fair equality should extent in actual practice.
The antiextensionists did not always employ scientific terms like "species" and "genus," but they nevertheless betrayed an implicit certainty that African Americans were humans, subject to the same privileges and responsibilities as all other humans. One Philadelphia columnist, writing under the pseudonym of "A Freeman," expressed his astonishment that slavery proponents would, "at this late day [maintain that] the right of one man to hold another in bondage was justifiable by the law of Nature and of God." A Vermont congressman argued "that a right to hold a fellow human being in slavery, under any term of Government, does not exist." ... However much the frequent omission of scientific terminology complicates the characterization of northern racial thinking in 1820, one critical consistency prevails: the restrictionists [opponents of slavery extension] acknowledged no inherent distinctions between men in formulating their defense of the slave's natural and religious rights. Given the improbability that these grounds could have been as liberally maintained in similar debates three decades later, the absence of a racialist idiom during the Missouri Crisis suggests that, to many northerners, it was still second nature to reiterate that''' God created all mankind equal, and endowed them with certain inalienable rights.'" [my emphasis]
Zeitz gives us a good example of that, too, which is worth quoting at some length to catch the complexity and contradictions involved:
The Baltimore newspaper editor Hezekiah Niles ruefully observed in 1819 that it was "the policy in some sections of our country, to keep the free people of color, as well as the slaves, in the grossest ignorance possible-to deprive them as far as practicable, of the capacity of reasoning and deny them the means of improvement." But Niles also expressed a cynical disdain for those starry-eyed abolitionists who "were clamorous for emancipation, without considering the consequences that must result from it." The successful manumission and cultural integration of the slave population clearly would require a preformulated plan, and that plan would rest on the assumption that the degeneracy of black America-both slave and free-was a direct but also reversible function of slavery. "[I]t is true wisdom," Niles explained, "to exalt the minds of slaves-to invest them with the correct ideas of [their] moral duties, and encourage them in the acquirement of property."There were many complicated elements to people's motivations on both sides of fight over chattel slavery. Some of them today look more noble than others. And some of the least noble ones still play an active role in American political life today.The states in which slavery is not allowed, should offer every reasonable facility and encouragement to free people of color ... and adopt some measures to lessen the prejudices and antipathies of the whites, in qualifying the blacks to attain a respectable standing in society.Lest his broad scheme for emancipation fall victim to skeptics, the editor explained two months later that "by adventitious mixtures, the effect of common association with the whites, and the operation of climate, the dark complexion [of the African American] might in time be nearly removed, if not wholly eradicated." This great amalgamation of the American races would be actuated "by the force of certain laws whose principles we do not understand, but whose effects are manifest throughout the works of animated nature." More importantly, it would occur "without sexual intercourse." [my emphasis in bold]