If Republicans lose control of either chamber of Congress in November, a constitutional crisis will erupt. A new Congress will ask questions about President Trump’s actions. The Trump administration will refuse to answer. The administration will argue that the Constitution grants Congress little power to order the executive to do much of anything. The executive, this administration insists, can do whatever isn’t explicitly forbidden; Congress can do only what is explicitly permitted. We know this will happen, because it’s already happening. The administration has lately been testing this “We can do anything” theory in the courts, including in the dusty domain of presidential recordkeeping. Thanks to the 1978 Presidential Records Act, every president from Ronald Reagan onward has been required by law to preserve and protect their records during their time in office, then turn them over to the National Archives when they leave. There are some caveats and exemptions, but the mandate has long been clear: Protect records during a presidency; archive them afterward. Few in 1978 or in the decades since have doubted …