Cite as: 600 U. S. ____ (2023) 23 Opinion of the Court drew no distinction between “procedural” and “substantive” restraints on lawmaking. It turned on the view that state constitutional provisions apply to a legislature’s exercise of lawmaking authority under the Elections Clause, with no concern about how those provisions might be categorized. 285 U. S., at 367–368; see also Hildebrant, 241 U. S., at 569–570. The same goes for the Court’s decision in Arizona State Legislature. The defendants attempt to cabin that case by arguing that the Court did not address substantive limits on the regulation of federal elections. But as in Smiley, the Court’s decision in Arizona State Legislature discussed no difference between procedure and substance. The dissent reads Smiley and Arizona State Legislature in a different light. JUSTICE THOMAS thinks those cases say nothing about whether a State can impose “substantive limits” on the legislature’s exercise of power under the Elections Clause. Post, at 21. But in Smiley, we addressed whether “the conditions which attach to the making of state laws” apply to legislatures exercising authority under the Elections Clause. 285 U. S., at 365. We held that they do. “Much that is urged in argument with regard to the meaning of the term ‘Legislature,’ ” we explained, “is beside the point.” Ibid. And we concluded in straightforward terms that legislatures must abide by “restriction[s] imposed by state constitutions . . . when exercising the lawmaking power” under the Elections Clause. Id., at 369. Arizona State Legislature said much the same, emphasizing that, by its text, nothing in the Elections Clause offers state legislatures carte blanche to act “in defiance of provisions of the State’s constitution.” 576 U. S., at 818. The defendants and JUSTICE THOMAS do not in any event offer a defensible line between procedure and substance in this context. “The line between procedural and substantive law is hazy.” Erie R. Co. v. Tompkins, 304 U. S. 64, 92 (1938) (Reed, J., concurring in part); see also Shady Grove