Gavin Newsom, meet John Calhoun. The Coast governor echoes the 19th-century backer of “nullification” by stoking defiance of President Trump’s tariffs. He vows to “fight back” against the new tariffs by forging “strategic partnerships with international trading partners” and pleading for “California-made products to be excluded” from any “foreign retaliatory measures.” Has Mr. Newsom forgotten his oath to the United States Constitution?

Among the bedrock features of the parchment are provisions prohibiting states from conducting their own foreign policy. Article I, Section 10, forbids any state, without the consent of Congress, to “enter into any agreement or compact” with “a foreign power.” Even more strict is the provision that “no state shall enter into any treaty, alliance, or confederation.” These provisions provide a test of the states’ commitment to one America.

So where does Mr. Newsom come off averring that “California is not Washington, D.C.”? That kind of reckless talk evokes the kind of secessionist fervor that has, occasionally in our history, reared its head. In the early 19th century, New England Federalists, enraged by federal limits on free trade, mooted secession. In 1861 the Civil War erupted in part — if only in part — due to Southern objections to high tariffs backed by the industrial North.

It recalls, too, the so-called “Nullification Crisis” that arose in 1832 after stiff protectionist tariffs were put up during Andrew Jackson’s presidency. A state that relied heavily on exports of cotton, South Carolina, balked at the economic damage of the tariffs. Vice President Calhoun hatched a theory of “interposition” that gave individual states the power to block a federal law unless the measure was imposed as an amendment to the Constitution.

An amendment, to be sure, is harder to achieve than passage of a law. An amendment requires passage by two-thirds of each house of Congress and three-fourths of states. In Calhoun’s calculation, this higher bar, giving states a de facto veto on federal law, was needed to protect the rights of political minorities and to avoid the tyranny of the majority. Old Hickory, though, wouldn’t stand for the Palmetto State’s pecksniffery.

At Jackson’s urging, Congress passed a law allowing him to send troops to South Carolina to collect the federal tariffs by force, if needed. “Disunion by armed force is treason,” he thundered. He balked at Calhoun’s Nullification theory, calling it a “strange position that any one state may not only declare an act of Congress void but prohibit its execution.” That, Jackson concluded, would give the states “the power of resisting all laws” — a recipe for chaos.

Has Calhoun’s 19th-century rallying cry for states’ rights found a 21st-century avatar in, of all people, the leftist Mr. Newsom? In a statement offered “to our international partners,” the governor crowed that “as the fifth largest economy in the world,” the Golden State would remain a steady, reliable partner for generations, no matter the turbulence coming out of Washington.” He touts his “state’s commitment to fair, open, and mutually beneficial trade.”

Mr. Newsom appears unabashed by the Constitution’s limits on state agreements with foreign governments. He crows that under his tenure, “California has signed 38 international agreements with 28 different foreign partners.” His predecessor, Governor Edmund “Jerry” Brown, too, “forged a series of climate change pacts” per Politico — with Communist China, no less. Mr. Brown saw “his actions as a counterweight to Trump’s intransigence,” adds Politico.

“We’re not scared to use our market power to fight back against the largest tax hike of our lifetime,” Mr. Newsom vows. His state “has the power to shape domestic manufacturing and international trade because of its enormous economy,” Politico says, and its trade ties to Asia and Mexico. Will a defiant Mr. Newsom eventually follow South Carolina’s example and claim that his state, “relying upon the blessings of God, will maintain its liberty at all hazards”?