In early October, President Trump’s White House counsel, Pat Cipollone, sent a defiant letter to four leaders of the House of Representatives. No one in the Trump administration, Cipollone declared, would participate in the impeachment inquiry that Speaker Nancy Pelosi opened in September after Trump’s phone call with President Volodymyr Zelensky of Ukraine came to light. “Because participating in this inquiry under the current unconstitutional posture would inflict lasting institutional harm on the executive branch and lasting damage to the separation of powers, you have left the president no choice,” Cipollone concluded.

Cipollone’s warning came at a pivotal moment for the balance of power between Congress and the presidency. Some political scientists have called Trump a weak president for failing to push through a legislative agenda. But in some ways, he has fit the model of an “imperial presidency,” historians say, by asserting in court — as no president has done before — that he is beyond the reach of criminal law. In a hearing before a federal appeals court in New York in October, one of Trump’s personal lawyers, William Consovoy, said that a sitting president cannot be held accountable for any crime, even if he shot someone. (“Local authorities couldn’t investigate?” Judge Denny Chin asked. “Nothing could be done? That is your position?” Consovoy answered, “That is correct.”) Trump’s Justice Department has also argued for vanishingly narrow readings, as applied to the president, of the crime of obstruction of justice and the Constitution’s emoluments clause, which was written to prevent foreign governments from exerting a corrupting influence.

Since the nation’s founding, Congress has exercised its oversight authority over the workings of the presidency by opening investigations, asserting its power to wrest information out of the executive branch. Its most important tool in this endeavor is the subpoena, which Congress can issue on its own, without the approval of a court, to interview witnesses and obtain documentary evidence. In 1821, the Supreme Court recognized Congress’s power to enforce its subpoenas by holding noncompliant officials in contempt, including by sending a sergeant-at-arms to arrest and detain them.

But over the last half century, the executive branch has grown bolder about withholding information in the name of national security or executive privilege, to keep the president’s communications confidential. And Congress has become the branch least able to enforce its own orders. It hasn’t sent a sergeant-at-arms to arrest or detain anyone since 1935. Asked to referee, courts tend to prefer compromise and negotiation. In 2008, the district court in Washington warned that “unseemly, provocative clashes” over locking up an executive-branch official “should be avoided.” Even when judges step in to take the side of Congress, the appeals process can take so long that the president wins simply by running out the clock. “The judgment is in Congress’s favor,” the Cornell law professor Josh Chafetz says. “But the political value of the information has reduced to zero.”