Remember how the Democratic Convention brought out the Gold Star father Khizr Khan to mock Donald Trump with the question, “Have you even read the United States Constitution?” It would be a good question for Michael Bloomberg. The ex-mayor of New York has just fetched up in the news as part of a campaign to undercut President Trump’s decision to pull out of the Paris climate accord.

It seems that His Honor is leading an effort to get not only corporations but also American states and cities to commit to an agreement America has just spurned. They are in negotiations with the United Nations. “We’re going to do everything America would have done if it had stayed committed,” Mr. Bloomberg is quoted by the New York Times as saying.

That would put Mr. Bloomberg on mighty thin constitutional ice. That’s because of the prohibition on any American state entering into any agreement or compact with another state or foreign power. That is American bedrock. It is contained in Article I, Section Ten, which is three paragraphs listing certain things that the American states may just never do.

It begins: “No State shall enter into any Treaty, Alliance, or Confederation.” The framers, the Heritage Foundation’s annotated edition of the parchment notes about this clause, “intended the Constitution to centralize much, if not all, power over foreign affairs.” Section Ten includes other prohibitions on the states. They mayn’t coin money, say, or grant any title of nobility or tax imports.

The section returns at the end to the topic here. It says that absent the consent of Congress “no state” shall “enter into any Agreement or Compact with another State, or with a foreign Power.” This language was stronger than the prohibitions that existed in the Articles of Confederation, which pre-dated the Constitution. The aim was to assure the federal government’s supremacy in international affairs.

What the Framers so clearly grasped is that, as Heritage puts it, “any foreign-affairs activities undertaken by the states” would be “so fraught with danger to the union, that none should be allowed unless Congress consented.” Does Section Ten give any pause to Mr. Bloomberg and his confederates treating with the U.N.? Do they comprehend the constitutional line they are in danger of crossing?

Mr. Bloomberg hasn’t responded to an email inquiry. But the Times quotes a draft letter to Secretary General Guterres from Mr. Bloomberg, saying: “While the executive branch of the U.S. government speaks on behalf of our nation in matters of foreign affairs, it does not determine many aspects of whether and how the United States takes action on climate change.”

“The bulk of the decisions which drive U.S. climate action in the aggregate are made by cities, states, businesses, and civil society,” the Times quotes the letter as saying. “Collectively, these actors remain committed to the Paris accord.” The paper reports that three Democratic governors have said that they are beginning “a separate alliance of states committed to upholding the Paris accord.”

There’s that word — “alliance.” It’s unclear whether the language the Times uses is from the governors or the Times’ own scribe. But any alliance by the states is prohibited by the plain language of Section Ten. Plus the Logan Act sets fines or imprisonment for any Yank who, without authority of the United States, carries on certain “correspondence or intercourse” with a foreign government.

Such intercourse is outlawed when the intent is “to defeat the measures of United States.” We respect Mayor Bloomberg’s idealism and willingness to commit his own cash. We’re merely marking the statutory language on citizen diplomacy and the Constitution’s bright lines on state-level action. The only state that is permitted to enter foreign agreements, treaties, alliances or compacts is the one headed by President Trump.