With a new Democratic House of Representatives, Donald Trump’s impeachment is for the first time a real possibility.

Already, Rep. Brad Sherman (D-CA) plans to introduce articles of impeachment against Trump Thursday. Don’t expect quick action on them, though. There are no signs that House Democratic leaders are seeking a battle over impeachment, and most rank-and-file members tend to treat the matter cautiously.

Yet this measured approach could change very quickly if new and damning information about Trump were to emerge from one of the many investigations into him or his inner circle: special counsel Robert Mueller’s Russia probe, investigations into Trump’s campaign hush money and inauguration, and the new probes that will soon be launched by Democratic House committee chairs.

Impeachment is much more likely now for a very simple reason: It only takes a majority vote in the House to impeach a president, and Democrats now have a majority.

But impeachment, should it happen, would only be the first step toward ousting Trump from office. To actually remove him, two-thirds of the Senate would have to vote to convict him — meaning at least 20 Republican votes would be needed.

It’s quite difficult to impeach, convict, and remove a president from office, so much so that’s it’s never happened in US history. (Two presidents have been impeached but acquitted; another resigned to avoid near-certain impeachment.)

If a sitting president of the United States stood in the middle of Fifth Avenue, shot a random person in broad daylight, and was caught with a smoking gun, it’s probably a safe bet that Congress would set aside partisanship to vote to impeach him, convict him, and remove him from office. (Probably.)

But most political scandals are not that indisputable, damning, or well-documented. And on any matter where there is some sort of plausible deniability for the president, his political allies will have very strong incentive to give him the benefit of the doubt, even if it means twisting themselves into knots.

The big picture is that rather than being run by any courts, impeachment and any ensuing presidential trial are carried out by the House of Representatives and the Senate, which are partisan bodies. So while it looks and feels a whole lot like a legal or judicial process, in practice impeachment is dominated by politics from start to finish.

What is impeachment?

The term “impeachment” itself dates back centuries in England, where it was “a device for prosecuting great lords and high officials who were beyond the reach of the law courts,” as David Stewart writes in Impeached, a book about President Andrew Johnson’s trial.

But in the US context, the framers of the Constitution set up the impeachment process to be a way Congress can remove the president from power.

First, the House of Representatives has the power to impeach the president. A simple majority is necessary for an article of impeachment to be approved (each article lays out a charge against the president).

Then the process moves to the Senate, where a trial will be held, with the chief justice of the Supreme Court presiding.

Finally, and crucially, it takes a two-thirds vote from the Senate to actually convict a president on any count. Conviction on any count would then remove the president from office and put the vice president in power.

Note that two-thirds of the Senate — 67 votes in the modern era — is a very high threshold that’s almost never achieved on any matter that’s remotely partisan. The framers did not make it easy for Congress to remove a democratically elected president from power.

What can the president actually be impeached for?

The Constitution specifies two specific crimes — treason and bribery — that could merit impeachment and removal from office. In addition to that, it mentions a vaguer, broader category of “other high Crimes and Misdemeanors.”

That’s all we get, and what, exactly, that last category entails has been the subject of a great deal of debate through US history. When Gerald Ford was House minority leader, he said, “An impeachable offense is whatever a majority of the House of Representatives considers it to be at a given moment in history.” But he said this when he was trying to impeach a Supreme Court justice, not a president.

Now, as a practical matter, Ford is absolutely right. If a majority of the House of Representatives wants to vote to impeach the president, it doesn’t seem that anyone can stop it from doing so. The Constitution says it has “the sole Power of Impeachment.” Unlike ordinary trials, evidentiary standards and even the charges themselves don’t necessarily have to be grounded in law; it’s all up to Congress to decide what matters.

Still, impeachment efforts that are wholly grounded in politics without even a thin pretext of an actual crime haven’t gotten very far, historically. In practice, some allegation of criminal behavior from the president has been necessary for the impeachment process to get moving — even if the true motivation for most of the primary actors really is political.

How often has impeachment happened in history?

The only two American presidents ever to have been impeached were Andrew Johnson in 1868 and Bill Clinton in 1998-’99. Neither was actually convicted and removed from office. However, Richard Nixon was headed toward seemingly certain impeachment and likely conviction in 1974, and preemptively resigned his office. No other president has come particularly close to being impeached.

1) Andrew Johnson: Johnson had been elevated to the presidency after Abraham Lincoln’s assassination, and his policy preferences about the post–Civil War South proved to be dramatically at odds with those of the Republicans who controlled Congress. (Essentially, Johnson was far more sympathetic to the interests of Southern whites, and wanted to restore them to power quickly in formerly Confederate states rather than pursue more extensive reforms.)

This years-long policy and political conflict was the true motivation for Johnson’s impeachment. The House of Representatives first considered whether to impeach Johnson in 1867, but voted against it — there was no consensus on what particular high crimes Johnson had committed.

Soon, though, they found their pretext. That same year, Congress overrode Johnson’s veto to pass a constitutionally dubious law called the Tenure in Office Act, which declared that the president could not fire his Cabinet officials without Senate approval. This was done to try to keep Johnson’s war secretary, Edwin Stanton, a staunch Republican ally whom Lincoln had appointed, in place to carry out Reconstruction policy. Still, Johnson fired Stanton anyway. The House responded by quickly impeaching the president, but he ended up being acquitted in the Senate by a single vote.

2) Richard Nixon: More than a century passed before the impeachment of a president was next seriously considered — in 1973, after revelations that President Nixon’s aides appeared to be complicit in the break-in at the Democratic National Committee offices in the Watergate Hotel and the cover-up that ensued.

As Dylan Matthews wrote in his comprehensive explainer, “It’s not really the break-in itself that ended Nixon’s presidency so much as the fact that the ensuing investigation revealed a tangled web of wrongdoing of almost unfathomable scale and complexity, implicating the highest levels of the White House, up to and including the president.”

Revelations of break-ins, efforts to smear political opponents or critics, cover-ups, attempts to suppress investigations, and hush money payments all came to light, leading to trials and convictions of several top Nixon aides. And crucially, the president taped himself discussing or approving many of these matters, and those tapes came to light — providing evidence that was tremendously difficult to dispute. By summer 1974, the House Judiciary Committee approved three articles of impeachment against Nixon (obstruction of justice, abuse of power, and contempt of Congress), and he resigned to preempt what seemed a near-certain impeachment.

3) Bill Clinton: Independent counsel Ken Starr had been operating a years-long investigation into various sprawling matters involving President Bill Clinton and his associates, and ended up filing a report alleging that Clinton had lied under oath and obstructed justice to prevent his affair with Monica Lewinsky from becoming known.

The House of Representatives voted to impeach Clinton for perjury and obstruction of justice, but the Senate acquitted him relatively easily — the president was popular, the impeachment effort was viewed as partisan, and enough senators concluded the crimes weren’t serious enough to warrant the president’s removal from office. The two-thirds Senate threshold for conviction remained far out of reach.

How does impeachment actually play out in the House?

Impeachment happens in the House of Representatives, and since the House is run on majority rules, it’s really up the majority party to run the process as it sees fit.

In our two 20th-century examples, Presidents Nixon and Clinton, the full House first voted to refer the matter to the House Judiciary Committee, which would draft impeachment articles and vote on whether to send them to the House floor.

In Nixon’s case, the House Judiciary Committee conducted its own investigation into Watergate and other scandals — holding hearings, hearing witness testimony, and so forth. Eventually, it approved three articles of impeachment, sending them to the House floor — but before the House could vote on them, Nixon resigned.

For Clinton, the Judiciary Committee decided not to conduct its own investigation, but rather to just vote on allegations drawn from the Starr report. The committee drafted four impeachment articles, and voted to approve them all.

Action over that next week in December 1998 then shifted to the floor of the House of Representatives. The full House voted on all four articles, with only a majority needed for approval of each. Two (a perjury count and an “abuse of office” count) were voted down, but the other two (one perjury count and one obstruction of justice count) were approved, though with overwhelmingly Republican votes and just a handful of Democrats. This meant Clinton was impeached, and that the Senate would have to hold a trial to determine whether to remove him from office.

Our older historical example, the 1868 impeachment of President Johnson, unfolded somewhat differently. When Johnson defied the Tenure of Office Act by dismissing his secretary of war without Senate approval, a special House committee on Reconstruction controlled by Johnson’s fiercest critics quickly voted to recommend impeachment. The full House of Representatives then voted overwhelmingly to impeach Johnson two days later — even though they didn’t have any specific charges drawn up yet.

A special House impeachment committee was created to actually draft those charges. Eventually, the full House voted to approve 11 specific articles of impeachment — most of which involved the Tenure of Office Act — though one focused on mean speeches the president gave disparaging Congress (seriously).

How does an impeachment trial play out in the Senate?

Though the actual action of impeachment in the House looks a lot like votes on any ordinary bill or resolution, the Senate is where things start to look quite different — because the Senate is hosting a trial, something it very rarely does.

In this trial, the House of Representatives acts as a prosecutor — designating certain impeachment managers to argue their side in the Senate. The president’s lawyers are the defense team — the president does not have to appear in person and historically has not. The chief justice of the Supreme Court presides and is responsible for making procedural rulings during the trial — however, the Senate can vote to overrule his decisions.

In addition to effectively being the jury that votes to acquit or convict at the end, senators get to decide how the trial is conducted. For Clinton’s trial, the Senate decided on a target end date, whether to allow House prosecutors to depose witnesses (the House wanted to depose 15, the Senate let them depose three), and whether to hear live witness testimony (they chose not to). Senators can make these decisions either through unanimous consent or by voting on a proposal put forth by Senate leadership.

Now, Johnson’s impeachment trial in 1868 looked a whole lot like a real trial. Witnesses were called, sworn testimony was given, and evidence was presented. The outcome truly was in the balance. At the end, senators had to vote on whether to convict or acquit on particular articles. A two-thirds vote on any one article would have convicted Johnson and remove him from office. However, in the three articles the Senate voted on, Republicans fell short of this margin by just a single vote all three times — so he was acquitted, and remained in office.

By contrast, Clinton’s impeachment trial in 1999 was a bit of a joke. It was clear to everyone in advance that Republicans weren’t even close to the two-thirds of senators they needed to convict Clinton, since the president was popular and the impeachment effort was viewed as partisan. As Jeffrey Toobin recounts in his book The Nine:

The Senate heard from no live witnesses, and the “trial” consisted almost entirely of statements by the House “managers” — the members of the Judiciary Committee who served as prosecutors — and Clinton’s defense lawyers. ... The dreary proceedings lasted five weeks. ... The outcome had never been in doubt.

During the trial, Toobin continues, Chief Justice William Rehnquist “made only a single substantive ruling” — he ruled that the House prosecutors shouldn’t keep referring to the senators as “jurors.” (Rehnquist later said, “I did nothing in particular, and I did it very well.”) In the end, Clinton was acquitted 55-45 on one count and 50-50 on the other, with Republicans not even coming close to the 67 votes they needed to remove him from office.

What does this mean for Trump?

Johnson, Nixon, and Clinton — the presidents who were either impeached or who, in Nixon’s case, resigned to preempt a coming impeachment — all faced Congresses in which both chambers were controlled by their political enemies, and who therefore wanted them out of power.

For Trump’s first two years, this wasn’t the case. His own party controlled the House and Senate. And since the president was quite popular among GOP voters, and party interests wanted Trump to appoint conservative judges and sign conservative bills, they had the strong incentive to give him the benefit of the doubt on any scandal or controversy. (Even if many of them might privately prefer it if Mike Pence were to become president.)

But now, Democrats will control the House. So the decision of whether to pursue Trump’s impeachment will really be up to them. And though some firebrands in the party have pushed for impeachment, Democratic leaders and most of the rank-and-file have been cautious about the prospect so far.

Hanging over this is the fact that winning the 20 or more Republican Senate votes that will be needed to actually remove Trump from office will be a very tall order. So Democrats have been wary about raising their voters’ expectations for a quest they fear will inevitably end in failure.

For instance, one key player will be Rep. Jerry Nadler (D-NY), chair of the House Judiciary Committee, which has a key role in impeachment. But Nadler has said that he fears impeachment will “tear the country apart,” and that he would only want to start the process if he thinks an “appreciable fraction of the Trump voters” could be convinced it was a good idea.

Still, all of that could change very quickly, should new information or evidence implicating Trump emerge from one of the many investigations against him. For instance, many expect that special counsel Robert Mueller has amassed a great deal of evidence that President Trump tried to obstruct justice.

Should Mueller complete a damning report on either obstruction or collusion with Russia, House Democrats could well be handed a ready-made case for impeachment. And they’d face enormous pressure from their base to take action on it. So buckle up.