President Trump speaks in the White House on Nov. 13. (Evan Vucci/AP)
David Priess is chief operating officer at the Lawfare Institute, a visiting fellow at the National Security Institute of George Mason University and a former CIA intelligence officer. His newest book is "How To Get Rid of a President: History’s Guide to Removing Unpopular, Unable, or Unfit Chief Executives."

The midterm elections have made it all but certain that some Democrats will introduce impeachment resolutions against President Trump after the new Congress begins in January. Speaker-in-waiting Nancy Pelosi (D-Calif.) downplays talk of impeachment and seems unlikely to use any such resolutions to initiate hearings or schedule an actual impeachment vote. But others are less circumspect.

On Sunday, Rep. Jerrold Nadler (D-N.Y.) — the likely incoming chairman of the House Judiciary Committee, traditionally responsible for impeachment hearings — told CNN’s Jake Tapper about Trump’s reported hush payments to women during the 2016 campaign: “It may be an impeachable offense if it goes to the question of the president procuring his office through corrupt means.” Rep. Adam B. Schiff (D-Calif.), poised to take the House Intelligence Committee helm, on the same day hinted to The Washington Post that Democrats would investigate Trump’s retaliations against media sources that have reported news about him that he doesn’t like as abuses of “instruments of state power.” His phrasing carries import; abuse of power has been a common charge across the many articles of impeachment introduced against previous presidents. Special counsel Robert S. Mueller III is expected to send Congress a report on his findings on Russian interference in the 2016 election and whether Trump attempted to block the investigation.

Already, more than three-quarters of self-identified Democratic voters in this month’s elections support impeachment, exit polls found. And they may well be right that Trump’s actions — on several fronts — could clear the threshold of “high crimes and misdemeanors.” But no one should suffer illusions about the likely result of any impeachment attempts.

Being deemed unfit for office — the condition intended by the Founding Fathers to trigger impeachment in the House — has never been enough to get the Senate to remove a president. History suggests that there wouldn’t be a successful conviction by two-thirds of senators without two other conditions in place: A chief executive must also be deeply unpopular. And booting him from office must seem more advantageous for the opposition in the next election than letting him remain there.

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“If he be not impeachable whilst in office,” William Davie told his fellow Constitutional Convention delegates in the summer of 1787 about the proposed president, “he will spare no efforts or means whatever to get himself reelected.” A couple of weeks later, Elbridge Gerry added his view of impeachments: “A good magistrate will not fear them. A bad one ought to be kept in fear of them.”

The delegates eventually agreed that an unfit president could be impeached (basically, indicted) for “Treason, Bribery, or High Crimes and Misdemeanors” by a majority vote in the House. Former professor of constitutional law Charles Black offers an admirable working definition of the latter phrase. “High crimes and misdemeanors,” he says, “ought to be held to those offenses which are rather obviously wrong, whether ‘criminal,’ and which so seriously threaten the order of the political society as to make pestilent and dangerous the continuance in power of their perpetrator.”

In today’s context, Black’s take on high crimes and misdemeanors is narrow enough to reject overzealous claims such as “Trump should be impeached because he is undermining America’s proven alliances,” but remains wide enough to encompass either the president’s attempts to curtail an investigation of himself or hush payments made to women in violation of campaign laws.

Although presidents have not been impeached as often as some founders probably would have predicted, two of the first 42 presidents did suffer that rebuke. In both cases, the required vote of two-thirds of senators to convict and remove the impeached chief executive, in a trial with the chief justice presiding, failed to materialize. History could repeat itself soon.

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Andrew Johnson, succeeding the assassinated Abraham Lincoln in 1865, quickly began provoking Congress by paroling leading Confederate officials, appointing governors for unreconstructed Southern states, and supporting those states’ right to institute discriminatory “black codes.” He routinely vetoed civil rights and other bills.

Overriding vetoes and other means of political obstruction weren’t enough for the so-called Radical Republicans on Capitol Hill. They wanted Johnson out of office — and they had a unique setup to do so. For the first time in American history, the president’s opposition held not only a majority in the House of Representatives but also a two-thirds majority in the Senate, sufficient for both impeachment and removal.

Lawmakers laid a trap. In February 1867, they overrode Johnson’s veto of the Tenure of Office Act, which required the Senate’s consent for the president to fire and replace identified executive branch officers, including the secretary of war — at that time Edwin Stanton, a strong advocate of U.S. military occupation of the South. On Feb. 21, 1868, Johnson removed Stanton, who refused to leave his office even to go home or to Cabinet meetings.

Capitol Hill erupted. The House held a quick vote on Feb. 24 and impeached the president. Johnson, however, managed to squeeze out exactly the number of votes he needed after the Senate’s two-month trial to prevent his removal.

Why didn’t the Senate remove this deeply unpopular president — and what does that tell us about a potential Trump trial?

First, the articles against Johnson were weakly drawn, mostly relying on his violation of the ill-conceived Tenure of Office law — hinting that any articles of impeachment against the president now must be based on established law, not trumped-up charges.

Second, in 1868, the president agreed to arrange jobs for a few undecided senators’ constituents and stop obstructing congressional Reconstruction plans — indicating that even if a majority vote in the House impeaches Trump, he could pick off some senators who remain on the fence and deflect a conviction.

Third, the law of the time directed that, if impeached, Johnson’s successor would have been Ohio’s Benjamin Wade — the Senate’s president pro tempore — who was, to put it mildly, unsuited for the presidency. (For years, he dared challengers to attack him in the Senate, having prominently placed two loaded pistols on his desk when he came into the chamber.) This suggests that if Trump were impeached, senators would be looking more carefully than they have to date at how Vice President Pence is comporting himself; if he fails any of their tests, Trump is on safer ground.

And fourth, many Republicans in 1868 weren’t looking for a post-impeachment wild card because they were already highly likely to take the White House within months with the election of the widely popular Ulysses S. Grant. Counterintuitively, then, the stronger the Democratic front-runners for the 2020 nomination look against Trump as the next presidential election approaches, the weaker the odds become of any post-impeachment removal before 2020.

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President Richard M. Nixon would not have resigned but for the sword of impeachment hanging over his head. The system ultimately worked: It got rid of the man who had proved himself unfit.

In late July 1974, after the Supreme Court ruled 8 to 0 that Nixon had to turn over tapes of White House conversations in which the president sought to obstruct justice, the House Judiciary Committee passed, with some Republican support, three articles of impeachment — covering obstruction of justice, abuse of power, and refusal to obey congressional subpoenas — and sent the articles to the full membership, along with a report of more than 500 pages.

On Aug. 20, 1974, the House accepted the impeachment report by a vote of 412 to 3. By then, however, Nixon had already given up — having resigned from office Aug. 9.

For those seeking to get Trump out of office, the lesson from Nixon’s era seems clear: They must focus on getting the president to see both that there is no viable escape route and that there is some inherent value in saving the country from the spectacle of the president being walked out of the Oval Office against his will after a guilty Senate verdict.

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The Republican majority in the House generally considered Bill Clinton’s behavior with White House intern Monica Lewinsky and his lying about it afterward both reprehensible and worthy of impeachment. But Democrats came to view it primarily as the case of an ashamed man trying to hide private sexual conduct — bad, no doubt, but not a constitutional travesty. The House in late December 1997 nevertheless passed two impeachment articles that focused on his perjury and obstruction of justice, shifting attention to the Senate. There, on Feb. 12, 1999, the articles didn’t even get a majority, much less the two-thirds needed for conviction.

Through it all, despite their solemn vow, each senator couldn’t help but have one eye on the polls. Even before the impeachment, 65 percent of Americans in a Gallup poll thought Republicans in Congress were attacking Clinton unfairly over a private matter; even 39 percent of Republicans agreed. Through the impeachment proceedings, the president’s approval ratings went up, not down.

Trump’s approval ratings, while low, have not eroded substantially since taking office. Instead, he maintains a solid support base that appears unwilling to ditch him lightly. That constituency may take an impeachment vote in the House as a shot of courage, which in turn could embolden Trump to hunker down and fight it out no matter what the damage to government institutions and norms.

It became hard for senators in the Clinton case to see the key issue for their vote as “Did he do the things he is accused of?” or “Are they high crimes and misdemeanors?” Many, if not most, instead saw a different question driving their votes, one we can imagine even more easily today: “Am I with this popular president or am I against him?”

To save himself from the indignity of impeachment, but probably also to keep a prolonged trial from paralyzing the country, Nixon ultimately resigned. If facing probable conviction and removal, we should hope that any president would do the same.

Without a dramatic turn of events, however, it’s unlikely that the Senate as currently composed would take Trump down even if the House were to impeach him. For Republican senators, his popularity among the party base will keep them in check. Many Democratic senators, for their part, probably relish the opportunity to use Trump’s record and his behavior in office so far against him in the 2020 presidential race and thus take back the White House.

Absent the president’s own party turning on him, which took down Nixon but seems unlikely in the foreseeable future, removal from office via impeachment will remain a steep mountain to climb — as kicking a president out without the direct input of the people should be.

This article is an adapted excerpt from “How To Get Rid of a President: History’s Guide to Removing Unpopular, Unable, or Unfit Chief Executives.”