While the first debate over slavery at the Constitutional Convention concerned representation (see Article I, Section 2, Clause 3), the second debate arose when Southern delegates objected that an unrestricted congressional power to regulate commerce could be used against Southern commercial interests to restrict or outlaw the slave trade. That the resulting provision was an important compromise is underscored by the fact that the clause stands as the first independent restraint on congressional powers, prior even to the restriction on the power to suspend the writ of habeas corpus.

Taking Southern concerns into consideration, the draft proposed by the Committee of Detail (chaired by John Rutledge of South Carolina) dealt with trade issues as well as those relating to slavery. The draft permanently forbade Congress to tax exports, to outlaw or tax the slave trade, or to pass navigation laws without two-thirds majorities in both houses of Congress. Several delegates strongly objected to the proposal, including Gouverneur Morris, who delivered one of the Convention’s most spirited denunciations of slavery, calling it a “nefarious institution” and “the curse of heaven.”

When the issue came up for a vote, the Southern delegates themselves were sharply divided. George Mason of Virginia condemned the “infernal traffic,” and Luther Martin of Maryland saw the restriction on Congress’s power over the slave trade as “inconsistent with the principles of the Revolution and dishonorable to the American character.” But delegates from Georgia and South Carolina announced that they would not support the Constitution without the restriction, with Charles Pinckney arguing that failing to include the clause would trigger “an exclusion of South Carolina from the Union.”

This same divide had also occurred in debating the Declaration of Independence. Though that document proclaimed the central principle of the Revolution that all men are created equal, a draft clause specifically condemning the slave trade was dropped based on Southern objections.

At the Constitutional Convention, the serious split caused by Southern insistence was referred to the Committee of Eleven (chaired by William Livingston of New Jersey), which took the opposite position from Rutledge and recognized a congressional power over the slave trade, but recommended that it be restricted for twelve years, though allowing a tax on slave importation. Although that was a significant change from the Committee of Detail’s original proposal, Southern delegates accepted the new arrangement but with the extension of the time period to twenty years, from 1800 to 1808.

Agitation against the slave trade was the leading cause espoused by the antislavery movement at the time of the Constitutional Convention, so it is not surprising that this clause was the most immediately controversial of the so-called slave clauses of the proposed Constitution (see also Article I, Section 2, Clause 3; Article IV, Section 2, Clause 3; and Article V). Although some denounced the Migration or Importation Clause as a major concession to slavery interests, most begrudged it to be a necessary and prudent compromise. Roger Sherman of Connecticut stated “better to let the southern states import slaves than to part with those states.” James Madison argued at the Convention that the twenty-year exemption was “dishonorable,” but in The Federalist No. 42, he declared that it was “a great point gained in favor of humanity that a period of twenty years may terminate forever, within these States” what he called an “unnatural traffic” that was “the barbarism of modern policy.”

In the North Carolina ratifying convention, James Iredell (later Supreme Court Justice) explained the reasons for the clause.

For my part, were it practicable to put an end to the importation of slaves immediately, it would give me the greatest pleasure; for it certainly is a trade utterly inconsistent with the rights of humanity, and under which great cruelties have been exercised. When the entire abolition of slavery takes place, it will be an event which must be pleasing to every generous mind, and every friend of human nature; but we often wish for things which are not attainable. It was the wish of a great majority of the Convention to put an end to the trade immediately; but the states of South Carolina and Georgia would not agree to it.

Even so, John Jay, later the first Chief Justice of the United States, thought that the clause had a limited geographical application, restricting Congress’s authority over the slave trade to the then existing states, but not to new states that might be admitted after ratification but prior to 1808.

Meanwhile, the states were left free to deal with the slavery question on their own. A number of states before 1800 did abolish slavery or provide for gradual emancipation. New Jersey, Rhode Island, and Connecticut had already abolished the slave trade; and New York and Delaware did likewise after the Constitution went into effect. Virginia declared that those slaves brought illegally into the state were free.

Although Southern delegates at the Constitutional Convention had hoped opposition to the slave trade would weaken with time, the practical effect of the clause was to create a growing expectation of federal legislation against the practice. The Slave Trade Act of 1794 prohibited any citizen or resident of the United States from carrying on any trade or traffic in slaves from the United States to any foreign country. Legislation in 1800 likewise prohibited engaging in such trade or traffic from one foreign country or place to another. Congress passed, and President Thomas Jefferson signed into law, a federal prohibition of the importation of slaves to the United States, effective January 1, 1808, the first day that Article I, Section 9, Clause 1 allowed such a law to go into effect.

Some observers at the time claimed that the Commerce Clause gave Congress the power to regulate both the interstate and the foreign slave trade once the twenty-year period had lapsed. James Wilson of Pennsylvania optimistically argued, “yet the lapse of a few years, and Congress will have power to exterminate slavery from within our borders.” Madison denied this interpretation during the First Congress. In Groves v. Slaughter (1841), the Supreme Court avoided the question whether slaves were articles of commerce for purposes of the Commerce Clause. Yet Abraham Lincoln did not claim that congressional power to regulate commerce could be used to restrict interstate commerce in slaves.

After 1808, the clause still had an effect on Supreme Court opinions. In Smith v. Turner (1849), Justice John McKinley opined (as James Iredell earlier declared in the North Carolina ratifying convention) that “migration” and “importation” referred to immigrants and slaves as distinct classes of people, and the clause therefore buttressed Congress’s exclusive power over immigration after the year 1808.

In Dred Scott v. Sandford (1857), Chief Justice Roger B. Taney pointed to this clause, along with the Fugitive Slave Clause (Article IV, Section 2, Clause 3), as evidence that persons of African descent were not ever intended to be accorded citizenship. Nevertheless, observers are virtually unanimous that those clauses did not address the question of citizenship at all. Although protection of the slave trade was a major concession demanded by proslavery delegates, the final clause was not a permanent element of the constitutional structure, but a temporary restriction of a delegated federal power. Moreover the restriction applied only to states existing at the time, not to new states or territories, and it did not prevent states from restricting or outlawing the slave trade for themselves. As the dissent in Dred Scott points out, there were free blacks who were citizens in a number of Northern states and who had voted to ratify the new constitution.

It is significant that the words slave and slavery are not used in the Constitution of 1787, and that the Framers used the word person rather than property. This would assure, as Madison explained in The Federalist No. 54, that a slave would be regarded “as a moral person, not as a mere article of property.” It was in the context of the slave trade debate at the Constitutional Convention that Madison argued that it was “wrong to admit in the Constitution the idea that there could be property in men.”