E: 8.32, S: -7.22
This is an extremely difficult issue to resolve, because neither the text nor the original understanding is clear. The Constitution does not expressly state that the power is exclusive; nor does it expressly forbid the states from exercising it. Thus, the question is, whether the power is of its very nature exclusive.
Justice Joseph Story seemed to believe that the power was exclusive. He wrote in his Commentaries:
"It has been settled upon the most solemn deliberation, that the power is exclusive in the government of the United States. The reasoning, upon which this doctrine is founded, is to the following effect. '... A grant of a power to regulate necessarily excludes the action of all others, who would perform the same operation on the same thing. Regulation is designed to indicate the entire result, applying to those parts, which remain as they were, as well as to those, which are altered. It produces a uniform whole, which is as much disturbed and deranged by changing, what the regulating power designs to have unbounded, as that, on which it has operated."
He proceeds to add that "The reasoning, by which the power given to congress to regulate commerce is maintained to be exclusive, has not been of late seriously controverted." And likewise, Justice William Johnson wrote in Gibbons:
"The power of a sovereign state over commerce ... amounts to nothing more than a power to limit and restrain it at pleasure. And since the power to prescribe the limits to its freedom, necessarily implies the power to determine what shall remain unrestrained, it follows, that the power must be exclusive; it can reside but in one potentate; and hence, the grant of this power carries with it the whole subject, leaving nothing for the State to act upon."
While the Marshall Court undoubtedly had a nationalist bent, their views were shared by the Republican James Madison. At the Convention, Madison said: "These terms ['to regulate commerce'] are vague but seem to exclude this power of the States." He was "more & more convinced that the regulation of Commerce was in its nature indivisible." I find it very difficult to question a judgment shared by a proponent of the states as prominent as Madison, and a proponent of the federal government as prominent as Story, both agree.
There appears to be strong evidence in favor of the view that the power is exclusive, but I must admit that I am not totally convinced.