State nullification is the idea that the states can and must refuse to enforce unconstitutional federal laws. Says Who? Says Thomas Jefferson, among other distinguished Americans. His draft of the Kentucky Resolutions of 1798 first introduced the word “nullification” into American political life, and follow-up resolutions in 1799 employed Jefferson’s formulation that “nullification…is the rightful remedy” when the federal government reaches beyond its constitutional powers. In the Virginia Resolutions of 1798, James Madison said the states were “duty bound to resist” when the federal government violated the Constitution. But Jefferson didn’t invent the idea. Federalist supporters of the Constitution...