Speaker of the House Nancy Pelosi, D-Calif., is surrounded by reporters as she arrives to meet with her caucus the morning after declaring she will launch a formal impeachment inquiry against President Donald Trump, at the Capitol in Washington, Wednesday, Sept. 25, 2019. Credit: J. Scott Applewhite | AP

House Speaker Nancy Pelosi, D-Calif., has announced a formal impeachment inquiry of President Trump. Here’s what you need to know about what impeachment is and how it works, starting with the basics.

What does impeachment actually mean?

It means that Congress thinks the president is no longer fit to serve and should be removed from office. Here’s what the Constitution says: “The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors.”

Who can impeach the president?

Congress. Specifically, the House of Representatives. Under the framework of the Constitution, the House can vote to impeach a president for “high crimes and misdemeanors.” It’s up to the House to decide what that means.

But impeaching the president is not the same thing as removing the president from office. For that, the Senate holds a trial presided over by the chief justice of the United States. Two presidents in American history have been impeached (Andrew Johnson and Bill Clinton). One resigned under threat of impeachment (Richard Nixon). None have been actually removed from office.

What is an impeachment inquiry?

It’s the first step in the impeachment process. It means lawmakers will investigate what, if any, “high crimes and misdemeanors” Trump may have committed.

What is the process for an impeachment inquiry?

Pelosi said Tuesday that the six key committees that are already investigating the president will continue to investigate Trump “under that umbrella of impeachment inquiry.” If the investigations conclude there are reasons for impeachment, the Judiciary Committee will draw up articles of impeachment, and the Judiciary Committee and then the full House will vote on it.

Here’s how Judiciary Committee Chairman Jerrold Nadler, D-N.Y., summed up the process in August to CNN, when he decided his committee was launching an impeachment inquiry: “We are investigating all the evidence, gathering the evidence. And we will [at the] conclusion of this – hopefully by the end of the year – vote to vote articles of impeachment to the House floor. Or we won’t. That’s a decision that we’ll have to make. But that’s exactly the process we’re in right now.”

So the House was already in an impeachment inquiry?

Yes. Well, kind of. It depends on whom you ask. Nadler’s committee has charge over impeachment, and he surprised some of his members this summer when he publicly said they’ve started an inquiry. Pelosi was not supportive of this and as recently as last week wouldn’t say “impeachment inquiry” publicly. But the allegations facing Trump on Ukraine changed her mind.

How many Democrats support an impeachment inquiry?

According to a Washington Post count, at least 218 House Democrats do, which is more than two-thirds of all House Democrats. That number is potentially significant because it takes 218 votes in the House to pass something. Not all of those 200-plus Democrats would vote for impeachment, though. It depends on what evidence the inquiry turns up.

Still, the number in favor of an impeachment inquiry has been growing steadily since April, when Special Counsel Robert Mueller released his redacted report on Russian election interference and Trump. That has been despite Pelosi’s efforts to tamp down on impeachment, because she feared it could cost vulnerable House Democrats their seats and maybe even cost the party the White House.

Since Congress came back from recess in September, the number of House Democrats who support an inquiry has ticked up by the week, sometimes by the day, and now, with the allegations Trump pressured Ukraine to help his reelection, by the hour. In the day before and on the day of Pelosi’s embrace of it Tuesday, 57 House Democrats on the fence came out in support of an impeachment inquiry into Trump.

How long does the impeachment process take?

It can be as long or as short as the House wants. As The Washington Post’s Aaron Blake notes, if past is precedent, this could be wrapped up in four months. But Democrats probably are on a tight timeline here; politically, it could be much more difficult to make their case that impeachment is necessary if it’s 2020 and nearing an election in which Trump could get thrown out of office anyway. As the Ukraine allegations were coming to light, a Quinnipiac University poll showed a majority of Americans, 57%, don’t support impeachment.

Will the Senate remove Trump from office?

As it stands now, probably not. There’s no evidence the Republican-controlled Senate wants to confront Trump in such a way. In fact, on Monday, Senate Republicans were trying to defend Trump. It’s up to House Democrats to uncover something that could change Republicans’ minds.

Another fascinating point on this: If the House impeaches Trump, it’s possible Senate Majority Leader Mitch McConnell, R-Ky., could just refuse to hold a trial. For centuries, the agreed-upon reading of the Constitution is that if the House impeaches a president, the Senate holds a trial to convict or acquit the president. But there could be some wiggle room for McConnell to avoid that spectacle, writes Bob Bauer, who served as White House counsel under Barack Obama.

What’s Trump saying about the impeachment inquiry?

He tweeted: “There has been no President in the history of our Country who has been treated so badly as I have. The Democrats are frozen with hatred and fear. They get nothing done. This should never be allowed to happen to another President. Witch Hunt!”

Could Trump run in 2020 even if he’s impeached?

If he’s impeached by the House, yes. If he’s removed from office, well, that’s never happened before, so we’d probably all be armchair-interpreting the Constitution to figure that one out.

The Washington Post’s JM Rieger contributed to this analysis.