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House Speaker Nancy Pelosi (D-Calif.) announced the launch of a formal impeachment inquiry into President Donald Trump on Tuesday over the president’s controversial phone call with Ukraine President Volodymyr Zelensky.

The phone call—made public by the White House Wednesday morning via a five-page “transcript”—caused the impeachment momentum to shift among the Democratic caucus.

So, what exactly is a formal impeachment inquiry? What are the odds of impeachment? And how is this different from past calls for Trump’s impeachment?

Here’s what we know so far. What do you want to know? Send politics editor Jaclyn Gallucci an email at jaclyn.gallucci@fortune.com. We’ll update this Q&A as things progress.

What is a formal impeachment inquiry?

An inquiry is the first step of any impeachment process. Pelosi, attempting to seize on a whistleblower complaint submitted to the Intelligence Community Inspector General about Trump’s call to Zelensky, is trying to hold the president accountable for possibly betraying “his oath of office.”

After an impeachment inquiry has been authorized, the House Judiciary Committee, can vote to send one or more articles of impeachment, or formal charges, to the House.

If a majority approves, it heads to the Senate.

Impeaching the president is not the same thing as removal. For removal, the Senate holds a trial presided over by the chief justice of the United States Supreme Court.

If two-thirds of the Senate votes for conviction, the official is removed from office.

What are the odds of Trump being impeached?

The odds that Donald Trump will be impeached and leave office before the end of his first term nearly doubled Wednesday, according to data compiled on wagers on the fate of the president placed by bettors from around the globe in the past 24 hours.

But since Republicans control the Senate, it’s still a big hill to climb. At least 20 Senate Republicans would need to join all 47 Senate Democratic Caucus members to actually remove Trump from office.

How many House Democrats are backing the impeachment inquiry?

It takes at least 218 votes in the House to move forward. By at least one count, more than 200 House Democrats support initiating impeachment proceedings as of Wednesday morning. But to push proceedings forward—and to potentially see Trump impeached and removed from office—those Democrats will need the support of Republicans, too.

Which Republicans might turn against Trump?

For now, most Republicans appear to be standing behind Trump, but that could change. Here’s a look at some of the Republicans who might already be wavering.

Vermont Gov. Phil Scott has supported the impeachment inquiry, but said he wants to know more before any additional action is taken.

“I think the inquiry is important, yes,” Scott said during a press conference Thursday, “and where it leads from here is going to be driven by the facts that are established.”

Sen. Mitt Romney (R-Utah) tweeted on Sunday that it “would be troubling in the extreme” if Trump “asked or pressured Ukraine’s president to investigate his political rival.” Speaking to NBC News’ Geoff Bennett Monday, Romney reportedly called for the whistleblower complaint to be made public, noting that if the White House doesn’t, “it will be up to the House to decide how to proceed.”

Sen. Cory Gardner (R-Colo.) told Politico that Trump’s conversations with the Ukrainian President and the whistleblower complaint are a “serious issue.” He refused to answer whether he still supports Trump’s re-election bid, saying, “Let’s find out what’s happening. Let’s get to the bottom of this.”

Sen. Susan Collins (R-Maine) has not endorsed Trump’s re-election but may face her own challenges in the 2020 election. She didn’t outline her position regarding impeachment to Politico, but did say that she doesn’t “know what evidence” the House is using, adding that she is still holding out hope “that we can legislate…but this could affect everything.”

While their positions will not affect the impeachment inquiry itself, there are at least a couple of Republicans who have expressed support for impeachment: Republican presidential candidates Joe Walsh (R-Ill.) and Bill Weld.

Meanwhile, George Conway, husband to White House counselor Kellyanne Conway, appeared to suggest Wednesday that Republicans will eventually change their tune on impeachment. “There may be Republican senators who won’t say a word until the moment they say ‘guilty’ when the roll is called at the end of an impeachment trial,” he tweeted.

Trump holds a 91% approval rating among Republicans, according to an early September Gallup poll, meaning that even if Congressional Republicans disapprove of him in private, it could be politically unwise for them oppose him publicly. There is one area where Senate Republicans are in lockstep with Democrats, however: the Senate unanimously voted Tuesday, calling on Trump to release the whistleblower complaint.

How will the impeachment inquiry affect the stock market?

With House Speaker Nancy Pelosi’s announcement of a formal Trump impeachment inquiry, stock markets don’t seem phased. After an opening drop, the S&P 500, Dow 30, Nasdaq, and Russell 2000 are all in positive territory.

But what about tomorrow, next week, month, and on into the coming year? If history is a tutor, things could go really well—or terribly, according to data compiled and sent to Fortune by LPL Financial.

What did Trump say in the call with Zelensky?

In Trump’s call with Zelensky, the president said he would like the Ukrainian president to “do us a favor” as he wanted to know what happened with “this whole situation with Ukraine,” with regard to CrowdStrike, a cybersecurity company that helped investigate Russian interference in the 2016 presidential election.

Trump also asked Zelensky to look into claims that former Vice President Joe Biden worked to remove a Ukrainian prosecutor specifically because he was probing a company where Biden’s son, Hunter Biden, worked.

Why did Trump mention internet security company CrowdStrike?

CrowdStrike is a U.S.-based internet security company that was hired to investigate the hack of the Democratic National Committee’s servers in 2016.

The company concluded that two groups compromised the same systems within the DNC network, but they “identified no collaboration between the two actors, or even an awareness of one by the other.” Despite this, CrowdStrike is confident that they were both working “for the benefit of the government of the Russian Federation and are believed to be closely linked to the Russian government’s powerful and highly capable intelligence services.”

CrowdStrike’s findings were later corroborated by several other independent cybersecurity firms. Yet Trump has repeatedly sought to cast doubt on the conclusions of these firms, pushing an unsubstantiated conspiracy theory that the DNC had hid one of its servers from the FBI, a server that reportedly has information about who was actually responsible for the hack.

What did the declassified whistleblower complaint say?

A redacted version of the declassified whistleblower complaint at the center of the impeachment inquiry was made public on Thursday morning.

We learned that in the days after the call, the whistleblower apparently learned from several U.S. officials that senior White House execs had been intervening to “lock down: all records of the phone call, especially the official word-for-word transcript of the call that was produced—as is customary—by the White House Situation Room,” the complaint said.

The whistleblower also said that White House officials were “directed” by the White House lawyers to remove the electronic transcript from the computer system where such transcripts are stored for coordination, finalizing, and eventually distributed to Cabinet-level officials.

Is the Ukraine call an impeachable offense?

Pelosi and dozens of House Democrats said that Trump abused his power with the call to Zelensky. Pelosi reiterated to reporters on Wednesday that the president used his office to put pressure on a foreign leader “at the expense of our national security as well as undermine the integrity of our elections…No one is above the law.”

During the call, Trump appears to demand support from Zelensky for his reelection campaign by trying to find derogatory information about Biden and his son, Hunter. The president asks repeatedly and, instead of asking Zelensky to work through official channels like the State Department, he would work with Trump’s personal attorney, former New York City Mayor Rudy Giuliani, who would be traveling to Ukraine.

Zelensky said he would meet with Giuliani during his visit and pledged that his new prosecutor would look into the case. Zelensky also asked for more information.

Impeachment, in essence, means that the U.S. Congress believes the president is no longer fit to serve and should be removed from office. According to the U.S. Constitution: “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.”

The House of Representatives, according to the Constitution, can vote to impeach a president for any “high crimes and misdemeanors.” What those crimes are, is up to the House.

How is this impeachment inquiry different from past impeachment calls?

Depending on who you ask, Trump has committed numerous impeachable offenses. So what’s different this time?

Despite a nearly two-year investigation by Special Counsel Robert Mueller into whether Trump’s campaign sought support from Russia during the 2016 election, the investigation’s findings were not plainly conclusive. This time around, Trump’s solicitation of Ukraine’s help is a little more clear-cut.

For one, Trump has admitted to the call with Ukrainian President Volodymyr Zelensky, and released a memo detailing their conversation, which shows that he asked the Ukrainian President to look into former Vice President Joe Biden and his son Hunter.

This makes the allegations of wrongdoing this time around a little clearer: Trump seemingly pressured a foreign leader to undermine one of his biggest opponents for the upcoming 2020 election. He simultaneously used U.S. military aid as leverage, initially withholding a $391 million aid package.

And while Trump shared the purported transcript of the call, he has thus far instructed the Director of National Intelligence not to share the whistleblower’s complaint on the matter with Congress, in clear disregard for the law.

This matter may be sufficiently straightforward and egregious to sway American public opinion, as Pelosi suggested on Tuesday. “This [allegation] is the most understandable by the public,” she said. “We don’t ask foreign governments to help us in our elections.”

Finally, this time around, the matter involves a sitting president. It is not a question of impeaching over past offenses from several years ago by candidate Trump, rather it involves an ongoing matter reportedly being undertaken by the current President of the United States.

As Council on Foreign Relations President Richard Haass puts it, “In 2016 @realDonaldTrump as private citizen could only ask Russia for help. Now he can use the powers of presidency to back his request of Ukraine, employing national security tools to back his political aims. This is basic difference betw 2016 and now & why this is more serious.”

How have Republicans responded?

So far, most Republicans who have made public comments about the impeachment inquiry are supporting the president. House Minority Leader Kevin McCarthy (R-Calif.) denounced Pelosi’s launch for an impeachment inquiry.

“Speaker Pelosi happens to be the speaker of this House but she does not speak for America when it comes to this issue,” McCarthy said Tuesday. “She cannot decide unilaterally what happens here.

McCarthy added, “I realize (the) 2016 (presidential election) did not turn out the way Speaker Pelosi wanted it to happen, but she cannot change the laws of this Congress. She cannot unilaterally decide we’re an impeachment inquiry.”

McCarthy remained firm with his stance when asked again by reporters on Wednesday.

On Thursday, Vermont Governor Phil Scott became the first GOP chief executive to support the inquiry, calling it “important,” but said he wants to know more before any more action is taken.

Meanwhile, a defiant Trump on Wednesday attacked Democrats and the media, calling them both corrupt.

“It’s the single greatest witch hunt in American history, probably in history, but in history,” Trump told reporters Wednesday at the United Nations Assembly meeting in New York City. “It’s a disgraceful thing. The letter was a great letter, meaning the letter revealing the call…It was a friendly letter, there was no pressure. The way you had that built up that call, it was going to be the call from hell. It turned out to be a nothing call, other than a lot of people said, ‘I didn’t know you could be so nice.'”

Who was the first U.S. president to be impeached?

Andrew Johnson, who became president after Abraham Lincoln was assassinated, was the first president to be impeached in American history in 1868. Johnson was impeached for firing Secretary of War Edwin Stanton, violating the Tenure of Office Act and vetoing several pieces of legislation that had been passed by Congress.

President Bill Clinton was impeached in 1998 by the House on articles charging perjury, specifically, lying to a federal grand jury, and obstruction of justice. The impeachment began in earnest because of Clinton lying about having a relationship with Monica Lewinsky, a 22-year-old White House intern. Clinton was acquitted by the Senate in 1999 and he went on to complete his first presidential term. Clinton was reelected when he defeated Bob Dole in 1996.

No presidents have been removed from office. President Richard Nixon, at the height of the Watergate scandal in 1974, resigned under threat of impeachment.