"Congress has no power to prohibit or obstruct . . ."

The eighth and final compromise resolution that Henry Clay introduced on January 29, 1850 is the least well known. It generally gets only a passing reference, at most. This is, presumably, because it neither proposed nor resulted in the passage of legislation. It merely declared that Congress had no power to “prohibit or obstruct” the interstate slave trade:

8th. Resolved, That Congress has no power to prohibit or obstruct the trade in slaves between the slaveholding States; but that the admission or exclusion of slaves brought from one into another of them, depends exclusively upon their own particular laws.

The resolution was presumably intended to reassure the south that, despite the 5th and 6th resolutions (which held that Congress had the power to regulate the slave trade in the District of Columbia and strongly implied that Congress had the power to abolish slavery there as well), Clay was not suggesting that Congress had similar power over the interstate slave trade under the Commerce Clause.

Clay's remarks on the resolution were brief. The resolution, he declared, “merely asserts a truth, established by the highest authority of law in this country, and in conformity with that decision I trust there will be one universal acquiescence.” Indeed, Clay maintained, the resolution was probably unnecessary,

but that I thought it might be useful in treating of the whole subject, and in accordance with the practice of our British and American ancestors, occasionally to resort to great fundamental principles, and bring them freshly and manifestly before our eyes, from time to time, to avoid their being violated upon any occasion.

Clay's allusion to “the highest authority of law in this country” must be a reference to the Supreme Court. He seems to have had a particular decision in mind, but I don't know what it was. If you do, please enlighten me in the comments!