This has given Cummings’s predecessors a unique ability to shape the public perception of recent presidencies — particularly when, as will be the case in January, it is a president of the other party. Many of the familiar details of the George W. Bush administration’s outing of the covert C.I.A. agent Valerie Plame, emergency-response failures during Hurricane Katrina and disastrous reconstruction of Iraq were ferreted out by Waxman, who led the Oversight and Government Reform Committee for the last two years of Bush’s second term. The congressional investigations of the Obama administration — into claims that the Internal Revenue Service targeted Tea Party groups and that the Justice Department allowed guns to be illegally trafficked across the Mexican border — that dominated Fox News chyrons after Republicans took back the House in 2011 were started by Waxman’s Republican successor, Darrell Issa.

As a rule, the Oversight and Government Reform Committee is a sleepy place when the same party controls Congress and the White House. But even by these standards, the committee’s performance during the first two years of the Trump administration has been unusual. Under the chairmanship of Representative Jason Chaffetz of Utah and then, after Chaffetz resigned in June 2017 and took a job at Fox News, Representative Trey Gowdy of South Carolina, the committee essentially turned a blind eye toward the executive branch. On matters big (like the firing of the F.B.I. director, James Comey, or the administration’s botched response to Hurricane Maria) and comparatively small (like Trump’s decision to revoke the security clearance of the former C.I.A. director John Brennan), the Oversight Committee did not seem interested in doing much real oversight.

“If the president’s party on Capitol Hill becomes subservient to the executive branch and just becomes an appendage of that, then Congress basically loses its meaning,” Tom Davis, the Virginia Republican who was chairman of the committee during part of Bush’s presidency, from 2003 to 2007, told me. “We turn into a parliamentary operation.” Norman Ornstein, a congressional scholar at the American Enterprise Institute, puts it even more bluntly. “Looking back over the first two years of the administration,” he says, “I can’t point to a single example, House or Senate, where any committee or subcommittee actually fulfilled its role of doing oversight.”

Over the past two years, as the committee’s ranking member, Cummings issued 64 subpoena requests; they were requests because the minority party can’t issue subpoenas without the majority’s approval. Chaffetz and Gowdy rejected them all. And even when Chaffetz or Gowdy did ask the Trump administration for information, they didn’t push very hard. “I was able to get them to jointly request documents that we needed to do our job,” Cummings told me, “but when the administration basically said, ‘Screw you’ — and the administration basically said that to every request — they refused to back it up with a subpoena.” Indeed, the first requests Cummings will send out as the incoming Oversight chairman — the letters about family separation, nongovernment emails and government-owned aircraft for personal travel — will be those that Chaffetz and Gowdy jointly sent with him over the past two years and that the administration largely ignored.

At the time, Cummings counseled his Republican colleagues to put aside the politics of the moment and to think long-term. “I get in these guys’ ears, and I talk to them heart to heart,” Cummings says. “I told them, ‘Trump is 72. I’m 67. You all are still young guys. Why are you trying to carry this guy’s water? After he’s dead, you’re going to be living with this [expletive].’ ”

Congress’s oversight responsibilities originated from an incident on Nov. 4, 1791, when a 1,400-soldier military expedition led by Arthur St. Clair, then governor of the Northwest Territory and a former general officer in the Continental Army during the Revolutionary War, was routed in what is now Ohio by a confederacy of warriors from three Native American tribes. Nearly 700 soldiers were killed and 300 wounded. The House of Representatives established a select committee to investigate the defeat and authorized it to “call for such persons, papers and records as may be necessary to assist their inquiries.” President George Washington was initially concerned that Congress had overstepped its bounds. But after Washington’s cabinet — including Secretary of State Thomas Jefferson and Treasury Secretary Alexander Hamilton — unanimously counseled him otherwise, he agreed to cooperate with the investigation, turning over the documents that had been requested.