Anyone who has studied any American history knows that slavery existed at the adoption of the U.S. Constitution in 1788 and that the Constitution’s original Article I, Section 2 apportioned representatives in the House of Representatives “according to their respective Numbers, which shall be determined by adding to the number of free Persons, including those bound to Service for a Term of Years, and excluding Indians no taxed, three fifths of all other Persons [i.e., slaves].” (Emphasis added.)
In addition, the First Congress in 1789 implicitly recognizing slavery when it enacted statues for an U.S. census and U.S. naturalization citizenship requirements. 
As the First Congress prepared to commence operations, emancipation advocates were seeking regulation of the slave trade or abolition of slavery while defenders of the “peculiar institution” contemplated secession if that happened. The latter’s House representation, of course, was bolstered by having their population increased by 60% (3/5th) of the number of their slaves.
The emancipation advocates were led by Quakers who starting in early February 1790 “way-laid” and “assailed” Senators and Congressmen with pamphlets and diagrams of overcrowded slaves ships while urging support of anti-slavery petitions. One such petition asked Congress “with a sense of religious duty” to end “the gross national iniquity of trafficking in the persons of fellow men” and “the inhuman tyranny and blood guiltiness inseparable from it.” Another petition that was signed by Benjamin Franklin called for use of “all justifiable measures to loosen the bonds of Slavery & promote a general enjoyment of the blessings of Freedom.”
Southern reaction to these petitions was “explosive.” They accused the Quakers of “intemperate and unwarrantable meddling,” of “an intolerant spirit of persecution” against the slave states, of disloyalty and cowardice during the Revolutionary War (because on religious principles they did not bear arms) and the promotion of “Insurrections & bloodshed & persecution.” A Georgia Congressman said religion “from Genesis to Revelations” had approved of slavery.
The three petitions were referred to a House select committee, which later reported that Congress had no power to emancipate slaves or interfere with the slave trade before 1808. On the other hand, the committee said, Congress had the power to put a tax on imported slaves and thereby motivate slave-owners and slave states to improve their treatment of slaves.
Thereafter the pro-slavery forces went on the attack. Their leader quoted Scripture, suggested that nothing could be done about it, that the new country needed exports to Africa and that slaves were incapable of mastering freedom. Some of the nation’s leaders personally opposed slavery—George Washington, Alexander Hamilton and James Madison-even though they were slave owners, but remained quiet in Congress because they feared loss of Southern support for other measures or a breakup of the new country.
The result? The House concluded that all power to end slavery and tax imported slaves rested with the states. In short, it was an endorsement of the status quo and the protection of slavery.
Fergus Bordewich, the author of a leading book on the First Congress, concludes that the “most consequential failure of the First Congress was its evasion of the corrosive problem of slavery. . . . Even members who loathed slavery feared that the new government could not risk an open debate on the subject without splintering . . . . [Thus,] for the next seven decades this evasion encouraged southerners to bully any northern politicians who challenged slavery by threatening secession and war, as the number of enslaved Americans swelled from 323,000 in 1790 to almost 4 million in 1861, and the moral problem of slavery became ever more deeply enmeshed with the politics of states’ rights.”
 The above provision of the original Constitution was deleted by Section 2 of the Fourteenth Amendment thereto, which was adopted after the Civil War in 1868 and which states: “Representatives shall be apportioned among the several States according to their respective numbers, counting the whole number of persons in each State, excluding Indians not taxed.”
 U.S. First Congress: Establishment of Racial Categories for the U.S. Census and U.S. Citizenship Naturalization, 1790, dwkcommentaries.com (Feb. 20, 2017).
 Bordewich, the First Congress at 3,6, 75-77, 104, 112, 124-25, 149, 151-52, 172, 178, 183, 195-96, 198-220, 223-24, 230, 244-45, 249, 276, 279-80 (Simon & Schuster, New York, 2016). http://www.simonandschuster.com/books/The-First-Congress/Fergus-M-Bordewich/9781451691931
 Id. at 304.