Kevin Lamarque / Reuters

Speaker Nancy Pelosi announced today that the House will launch an impeachment inquiry into President Donald Trump. The move follows a sudden shift in the Democratic caucus over the past week, as allegations that the president pressured Ukraine to boost his reelection prospects swirled. Many previously reticent Democrats, chief among them Pelosi herself, have changed their mind and now support an inquiry.

To a great extent, today’s announcement seems to have been inevitable since November 2018, when Democrats won control of the House; or since May 2017, when Trump fired FBI Director James Comey; or even since November 2016, when Trump was elected and horrified Democrats began plotting maneuvers to stop him. The question was always when a critical mass of Democrats would coalesce in favor.

But if the path to impeachment was predictable, the path from here is not. History suggests that once investigations into presidents begin, they tend to head in unexpected directions.

Speaking briefly late this afternoon, Pelosi accused Trump of repeatedly breaking the law and violating the Constitution.

“The actions of the Trump presidency revealed dishonorable facts of betrayal of his oath of office and betrayal of our national security and betrayal of the integrity of our elections,” she said. “Therefore today I’m announcing the House of Representatives is moving forward with an official impeachment inquiry.”

She added: “The president must be held accountable. No one is above the law.”

The rhetorical stakes of that comment are high; the material stakes are not so clear. Pelosi’s announcement came after months in which she worked to put the brakes on impeachment, even as a growing number of Democrats began to support launching a formal inquiry. But over the past few days, more members of the caucus joined them. That included a group of moderate Democratic freshmen, whom Pelosi feared she would be endangering politically by moving forward, and Representative John Lewis, a respected elder statesman.

But although Pelosi called it an “official impeachment inquiry,” she didn’t offer any indication of what that would mean in practice. She said she is directing chairs of committees to work together “under that umbrella of impeachment inquiry.” She did not announce plans for a separate impeachment inquiry, nor for new appropriations or hiring. That creates the possibility that business will continue in the House as usual, only with the speaker herself now labeling that business an impeachment inquiry.

“If we have to honor our oath of office to support and defend the Constitution of the United States, from all enemies, foreign and domestic, that’s what we’ll have to do,” Pelosi told The Atlantic’s editor in chief, Jeffrey Goldberg, today at The Atlantic Festival in Washington. “But we have to have the facts.”

The protean nature of impeachment inquiries became apparent in the first attempt to remove a president. The House began considering impeaching Andrew Johnson in early 1867. As my colleague Yoni Appelbaum has written, the initial charges against Johnson included a host of substanceless rumors, including the allegation that he’d been involved in the plot to assassinate Abraham Lincoln. As the investigation proceeded, a number of rumors were exposed as baseless. “Many of the claims against Johnson failed to survive the journey,” Appelbaum writes. “Those that did eventually helped form the basis for his impeachment.”

The act that ultimately led to Johnson’s impeachment, the firing of Secretary of War Edwin Stanton, had not yet occurred when the impeachment inquiry began. But once an inquiry was open, it took on a life of its own, and ultimately brought the president to within a single vote of removal from office.

Richard Nixon was not impeached, because he resigned from office before the House had a chance to act, realizing he would probably be convicted and removed from office. Still, the events that led up to his resignation demonstrate the same unpredictability. Some House Democrats had been toying with impeachment even before the Watergate break-in. There were new calls, again unsuccessful, after the break-in. Only after the Saturday Night Massacre did the House Judiciary Committee open an impeachment inquiry. Meanwhile, the Senate Watergate Committee, which had been created to investigate the break-in but did not assume there had been presidential wrongdoing, turned up extensive evidence of Nixon’s personal involvement in a cover-up. One of the three articles of impeachment passed against Nixon concerned his refusal to comply with subpoenas by the committee after the impeachment inquiry had begun.

The case of Bill Clinton, the second U.S. president to be impeached, is slightly different. By the time the House Judiciary Committee began its impeachment inquiry, the focus had been narrowed to Clinton’s handling of his affair with an intern. Yet the Clinton example also confirms that investigations can start in one place and end in another. Independent Counsel Ken Starr was originally appointed to investigate the Whitewater scandal. While Starr charged several people with crimes, neither Bill nor Hillary Clinton was accused of wrongdoing in Whitewater. Once Starr was investigating, however, he turned up a variety of other matters—most consequentially Clinton’s affair with Monica Lewinsky. He turned evidence over to Congress, which then impeached the president. The Senate did not convict.

In contrast to Starr’s expansive investigation, Special Counsel Robert Mueller kept a narrower focus when he was appointed to investigate Russian interference in the 2016 election. Mueller probed Russian actions, the Trump campaign’s responses, and then the president’s post-election efforts to stifle the investigation, but he also referred other matters, such as campaign-finance violations, to other prosecutors.

A House impeachment inquiry led by Democrats is not likely to place such constraints on itself, which is why it could expand or take unpredictable turns, just as the Johnson and Nixon investigations did. Each time a new Trump scandal arises, the question has been whether it is the one that will push Democrats over the top to begin an impeachment inquiry. Until today, none had done the trick. Once the inquiry is open, however, the committee can and probably will vacuum up any number of other matters—past, present, and future. The barrier to entry has been lowered.

As Susan Hennessey, Quinta Jurecic, and Benjamin Wittes write at Lawfare, “Trump’s misconduct presents what the military calls a target-rich environment. There’s a huge range of activity that a reasonable member of Congress could in good conscience regard as impeachable.”

Of course, every instance of misbehavior that comes to Democrats’ attention does not rise to the level of impeachment; even among those that do, there could be political and logistical reasons to steer clear of them. Still, the formalization of an inquiry that could lead to impeachment means there’s a committee ready for whatever might come across its desk. That makes the future path of the inquiry complicated and unpredictable.

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