(Reuters) - Impeachment of a U.S. president does not mean an immediate ouster from office, or as House of Representatives Speaker Nancy Pelosi put it on Wednesday, a “bye, bye birdie.”

FEBRUARY 5, 2019 - WASHINGTON, DC: President Donald Trump delivered the State of the Union address, with Vice President Mike Pence and Speaker of the House Nancy Pelosi, at the Capitol in Washington, DC on February 5, 2019. Doug Mills/Pool via REUTERS

Pelosi is under pressure from some Democrats to begin an impeachment of Republican President Donald Trump, who last week called impeachment “a dirty, filthy, disgusting word.”

She has tried to tamp down Democrats’ demands to move against Trump and she talked to reporters about the issue.

“Do you know most people think that impeachment means you’re out of office?” she asked reporters. “They think that you get impeached, you’re gone. And that is completely not true ...

“It’s not the means to the end that people think - all you do is vote to impeach, bye bye birdie. It isn’t that.”

The U.S. Constitution empowers Congress to impeach and remove the president from office, but no president has ever been removed as a direct result of a Senate impeachment trial.

The House is now controlled by Democrats. So they might be able to pass an impeachment resolution. But the Senate, where an impeachment would have to be adjudicated, is controlled by Republicans, who are unlikely to vote to remove Trump.

Here is how the impeachment process works.

WHY IMPEACHMENT?

The founders of the United States created the office of the presidency and feared that its powers could be abused. So they included impeachment as a key part of the Constitution.

They gave the House “the sole power of impeachment;” the Senate “the sole power to try all impeachments;” and the chief justice of the Supreme Court the duty of presiding over impeachment trials in the Senate.

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The president, under the Constitution, can be removed from office for “treason, bribery, or other high crimes and misdemeanors.” What exactly high crimes and misdemeanors means is unclear. Historically, it can encompass corruption and other abuses, including trying to obstruct judicial proceedings.

House Democrats are investigating whether Trump tried to obstruct Special Counsel Robert Mueller’s inquiry into Russian interference in the 2016 U.S. presidential election campaign.

A redacted version of Mueller’s final report, released in mid-April, outlined multiple instances in which Trump tried to thwart the probe. While it stopped short of concluding Trump had committed a crime, the report did not exonerate him.

“In terms of the crimes committed, we know that he obstructed justice,” Pelosi said on Wednesday.

HOW DOES IT WORK?

Impeachment begins in the House, which debates and votes on whether to bring charges against the president via approval of an impeachment resolution, or “articles of impeachment,” by a simple majority of the House’s 435 members.

If the House approves such a resolution, a trial is held in the 100-member Senate. House members act as the prosecutors; the senators as jurors; the chief justice of the Supreme Court presides. A two-thirds vote of the senators is needed to convict and remove a president from office. This has never happened.

Presidents Andrew Johnson in 1868 and Bill Clinton in 1998 were impeached by the House, but both of them remained in office after being acquitted by the Senate. President Richard Nixon resigned in 1974 before Congress could impeach him.

SUPREME COURT

Trump has said on Twitter that he would ask the Supreme Court to intervene if Democrats tried to impeach him. But the founders explicitly rejected making a Senate conviction appealable to the federal judiciary.

STANDARDS OF PROOF

In a typical criminal court case, jurors are told to convict only if there is “proof beyond a reasonable doubt,” a fairly stringent standard. Impeachment proceedings are different. The House and Senate can set their own standards for proof.

Before he became president in 1974, Republican Vice President Gerald Ford said: “An impeachable offense is whatever a majority of the House of Representatives considers it to be at a given moment in history.” Ford replaced Nixon.

PARTY BREAKDOWN

The House has 235 Democrats, 197 Republicans and three vacant seats. As a result, the Democrats could impeach Trump with no Republican support.

In 1998, when Republicans had a House majority, the chamber voted largely along party lines to impeach Clinton, a Democrat.

The Senate now has 53 Republicans, 45 Democrats and two independents who usually vote with the Democrats. Conviction and removal of a president would require 67 votes. So, for Trump to be removed from office, at least 20 Republicans and all the Democrats and independents would have to vote to convict.

WHO WOULD REPLACE TRUMP?

In the unlikely event the Senate convicted Trump, Vice President Mike Pence would become president for the remainder of Trump’s term, which ends on Jan. 20, 2021.