One interesting question lurking in the background of some recent Supreme Court cases is the scope of discretion executive agencies receive when they assert very broad power over areas that are traditionally committed to state authority. Traditionally, under the Court’s landmark 1984 decision in Chevron v. Natural Resources Defense Council, courts are supposed to defer to an agency’s “reasonable” interpretation of its authority when the statute does not clearly speak to the question at issue. The question is, what happens when an ambiguous statute buts up against the outer boundaries of federal constitutional power?

After Gonzales v. Raich, the question was all the more pressing. Raich held that Commerce Clause concerns are at a low ebb when Congress regulates an interstate market “comprehensively.” In that context, when Congress finds that regulating local conduct is essential to the larger regulatory scheme, the majority in Raich said it will defer to Congress’s judgment. As Cato’s amicus brief in Rapanos argued:

If, after Raich, agencies can use legislative history, statutory purpose, or context to manufacture ambiguity nowhere apparent from the text of a statute, and if, in turn, agencies interpreting their power under “ambiguous” statutes are granted both the full quantum of deference owed to Congress under Raich and under Chevron, the potential for agency aggrandizement is immense, indeed.

In light of these and other concerns, we argued, Congress (at a bare minimum) must clearly state in the text of the statute that it intends to push the envelope of federal power before agencies can assume the power and discretion to step into an area traditionally regulated by states.

Scalia appears to take these concerns seriously. First, he notes: “Even if the phrase ‘the waters of the United States’ were ambiguous as applied to intermittent flows, our own canons of construction would establish that the Corps’ interpretation of the statute is impermissible.” In other words, the Court’s constitutional concerns trump agency discretion under an ambiguous statutes–even one that constitutes comprehensive regulation under Raich. This is an important qualification of the traditional Chevron test, one that reigns in the worst excesses of Raich.

Second, in footnote 9, Scalia underscores that before an agency can reach local conduct under a comprehensive regulatory program, Congress must clearly authorize it to do so in the statutory text. Only then will the Court consider whether the agency’s authority is consistent with the Constitution’s division of power between the federal government and states.

The troubling thing about Roberts’ concurrence is that, on one reading, he would appear to take a far more expansive view of agency discretion. He says:

Agencies delegated rulemaking authority under a statute such as the Clean Water Act are afforded generous leeway by the courts in interpreting the statute they are entrusted to administer. See Chevron U. S. A. Inc. v. Natural Resources Defense Council, Inc., 467 U. S. 837, 842–845 (1984). Given the broad, somewhat ambiguous, but nonetheless clearly limiting terms Congress employed in the Clean Water Act, the Corps and the EPA would have enjoyed plenty of room to operate in developing some notion of an outer bound to the reach of their authority.

The proposed rulemaking went nowhere. Rather than refining its view of its authority in light of our decision in SWANCC, and providing guidance meriting deference under our generous standards, the Corps chose to adhere to its essentially boundless view of the scope of its power. The upshot today is another defeat for the agency.

Its far from 100% clear what kind of test Roberts envisions here. But the best reading is this: If the agency deliberates about its constitutional and statutory authority in a “limiting way” in the context of public notice and comment procedures, he would give the agency “generous” deference, even the EPA would draw lines different (and more expansive) than those that Scalia’s plurality opinion draws. In effect, Roberts would give an agency treading close to the constitutional boundary the same deference that the Court grants to Congress under Raich if the agency draws any limits, no matter how slight, on its authority–precisely the interpretive method we raise red flags about in our brief.