WASHINGTON – President Donald Trump now faces a formal impeachment inquiry. But that doesn’t necessarily mean he’ll be removed from office.

Impeachment, embedded in the constitution, is a lengthy, complicated process.

House Speaker Nancy Pelosi, under pressure from some Democrats to start impeachment proceedings, announced Tuesday the opening of an impeachment inquiry against Trump amid revelations he urged Ukraine to investigate his political rival, Joe Biden.

Pelosi understands the challenges Democrats face in proceeding with impeachment proceedings and has spent months trying to swat away such talk whenever it comes up.

But the revelations about Trump's phone call about Biden apparently were the tipping point, with several Democrats who had been on the fence now in favor of opening an impeachment inquiry.

Here are a few things to know about the impeachment process and how it works:

What is impeachment?

Americans often equate impeachment with the removal of a president or other federal officials from office for committing a crime.

But that’s incorrect.

Impeachment is nothing more than the approval of formal charges against a president, vice president or other federal officeholder who stands accused of committing a crime. It’s like an indictment in a criminal proceeding.

Once an officeholder is impeached by the House of Representatives, a trial is held by the Senate to determine whether the accused is guilty of the charges. If a guilty verdict is returned, only then can the accused be removed from office.

Why is Trump under fire?

Trump’s admission that he urged Ukraine to investigate Biden has set off new calls for the president's impeachment.

The controversy over Ukraine began with reports that a whistleblower in the intelligence community called attention to conversations between Trump and a foreign leader. Trump acknowledged calling Ukraine’s president, Volodymyr Zelensky, on July 25 to urge him to fight corruption and investigate Biden’s son, Hunter Biden, who served on the board of an energy company.

Trump also withheld military aid for Ukraine, although he insisted he didn't use the money as leverage to demand an investigation by Kiev into Biden. Democrats, however, argued the demand to investigate Biden sound like extortion.

Facing increasing pressure, Trump said Tuesday he has authorized the release of a transcript of the call.

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Some Democrats have been pushing for months for Pelosi to open an impeachment inquiry, even before the latest controversy involving Ukraine.

Impeachment chatter started earlier this year as special counsel Robert Mueller was investigating Russian interference in the 2016 election and whether anyone in Trump’s campaign conspired with Moscow.

Mueller’s long-awaited report, released in April, detailed multiple contacts between Russian operatives and Trump associates during the campaign, but investigators didn’t find any evidence of a criminal conspiracy.

The report did document a series of actions by Trump to derail the special counsel's investigation and cites 10 instances of possible obstruction by the president. Mueller’s office did not conclude Trump's actions were illegal but also pointedly refused to clear him of wrongdoing.

Some House Democrats, however, seized on what they called the "damning details" against Trump and argued his actions were illegal and should be the basis to begin impeachment proceedings. Pelosi resisted, and the impeachment talk died down until the latest revelations came to light.

Tipping point?:House Democrats debate impeachment over Ukraine allegations

Who files impeachment charges?

The U.S. Constitution gives the House the sole power to impeach a federal official. Individual House members can introduce impeachment resolutions like ordinary bills, or the House could initiate impeachment proceedings by passing a resolution authorizing an inquiry.

The House Judiciary Committee, which has jurisdiction over impeachment proceedings, decides whether to pursue articles of impeachment against the accused official and can hold hearings to review the accusations. If the committee votes to approve articles of impeachment, the formal charges move to the full House for consideration.

The House votes on each of the articles of impeachment separately and can approve them with a simple majority of those voting. If all 435 House members vote, then 218 votes would be needed.

If the articles are adopted, the accused has formally been impeached.

If the House were to initiate impeachment proceedings, it’s certainly possible they would have the votes to approve impeachment articles against Trump. Democrats hold a 235-198 majority in the House, giving them far more than the 218 votes needed to impeach.

In the Senate, however, Republicans hold a 53-47 advantage over Democrats. To remove Trump from office, Democrats would have to persuade at least 20 Republicans to vote for his conviction. That won’t happen unless something extraordinary happens to cause his support to dwindle among GOP senators.

More:What's going on with Trump and Ukraine and a whistleblower complaint?

What happens after an impeachment?

Once an officeholder has been impeached, the proceedings shift to the Senate, which holds a trial and decides whether to convict the accused and remove him or her from office.

The House appoints members to manage the trial and act as prosecutors in the Senate. The Chief Justice of the Supreme Court presides. At the end of the trial, which can last for weeks or even months, the Senate votes whether to convict the accused on each of the charges.

A two-thirds majority, or 67 senators, is needed to convict and remove the accused from office.

Biden weighs in:Former VP to call for impeachment if Trump doesn't comply with Congressional inquiries

What's an impeachable offense?

Article II, Section 4 of the Constitution says the president, vice president and other federal officials can be removed from office upon conviction for treason, bribery and “other high crimes and misdemeanors.”

But what are “high crimes and misdemeanors?”

“This is one of the most famously contested phrases in all of constitutional law,” said Joshua Matz, one of the authors of the book “To End a Presidency: The Power of Impeachment.”

“Simply put,” Matz said in an interview with USA TODAY, “impeachable offenses involve an abuse of power, a betrayal of the nation or a corruption of the office for private benefit that poses a substantial and ongoing risk of harm to the ongoing constitutional order if the president is allowed to remain in office.”

Under that standard, a president can’t be impeached simply over policy disagreements, partisan differences, negligence or recklessness, or for making poor personnel decisions.

“The president has to have done something bad, very bad,” Matz said.

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Who has been impeached?

The House has initiated impeachment proceedings more than 60 times, but less than a third have led to full impeachments, according to the House’s History, Art and Archives website.

Just eight of those impeached – all federal judges – have been convicted and removed from office by the Senate.Officeholders impeached but not removed include a cabinet secretary (Secretary of War William Belknap in 1876), a U.S. senator (William Blount of North Carolina in 1797) and two presidents.

President Andrew Johnson was impeached in 1868 after a political conflict with Congress involving his Reconstruction policies after the Civil War and his removal of Secretary of War Edwin Stanton. Thirty-five senators voted to find him guilty of the charges – a single vote short of the two-thirds majority required for conviction and removal from office.

More than a century later, President Bill Clinton was impeached for perjury and obstruction of justice in the wake of his extramarital affair with White House intern Monica Lewinsky. The Senate failed to muster the two-thirds majority for a conviction on either of the charges, and Clinton went on to serve the remainder of his second term in office.

Looking back on Clinton:Lesson to learn from his impeachment, 20 years later

President Richard Nixon was on the verge of impeachment in 1974 over his role in the Watergate scandal. The House Judiciary Committee approved three articles of impeachment against him alleging obstruction of justice, abuse of power and contempt of Congress. But Nixon resigned from office before the charges were taken up by the full House.

Can impeachment be appealed?

When impeachment talk was swirling around Trump following the release of the Mueller report, Trump suggested he’d ask the Supreme Court to intervene if Democrats tried to impeach him.

Matz said the comment shows Trump has “a fundamental misunderstanding” of the Constitution and how the impeachment process works.

While the chief justice presides over an impeachment trial in the Senate, the framers of the Constitution explicitly refused to give the other justices a role in the process, he said, in part because they feared a president would pack the courts with justices who share his world view and "they didn’t want a president to choose his own judge in an impeachment."

Nearly three decades ago, a federal judge from Mississippi asked the courts to overturn his perjury conviction and removal from office. But in a 1993 decision written by Chief Justice William Rehnquist, the Supreme Court held that the courts may not review such cases because the Constitution makes it clear the House has the sole power of impeachment and the Senate has the sole power to try impeachment articles.

"That phrase – 'sole power' – at the very least indicates an overwhelming presumption against any Supreme Court involvement in the substance of an impeachment proceeding," Matz said.

In other words, "President Trump’s apparent belief that the justices he appointed to the Supreme Court will save him from impeachment is gravely misguided," he said.

Contributing: David Jackson, John Fritze