(Reuters) - The “i word” - impeachment - is swirling around the U.S. Congress since the release of Special Counsel Robert Mueller’s redacted Russia report, which painted a picture of lies, threats and confusion in Donald Trump’s White House.

FILE PHOTO: U.S. President Donald Trump waves to his audience as he hosts Take Our Daughters and Sons to Work Day at the White House in Washington, U.S., April 25, 2019. REUTERS/Kevin Lamarque

Some Democrats say trying to remove Trump from office would be a waste of time because his fellow Republicans still have majority control of the Senate. Other Democrats argue they have a moral obligation at least to try to impeach, even though Mueller did not charge Trump with conspiring with Russia in the 2016 U.S. election or with obstruction of justice.

Whether or not the Democrats decide to go down this risky path, here is how the impeachment process works.

WHAT ARE GROUNDS FOR IMPEACHMENT?

The U.S. Constitution says the president can be removed from office by Congress for “treason, bribery, or other high crimes and misdemeanours.” Exactly what that means is unclear.

Before he became president in 1974, replacing Republican Richard Nixon who resigned over the Watergate scandal, Gerald Ford said: “An impeachable offence is whatever a majority of the House of Representatives considers it to be at a given moment in history.”

Frank Bowman, a University of Missouri law professor and author of a forthcoming book on the history of impeachment, said Congress could look beyond criminal laws in defining “high crimes and misdemeanours.” Historically, it can encompass corruption and other abuses, including trying to obstruct judicial proceedings.

HOW DOES IMPEACHMENT PLAY OUT?

The term impeachment is often interpreted as simply removing a president from office, but that is not strictly accurate.

Impeachment technically refers to the 435-member House of Representatives approving formal charges against a president.

The House effectively acts as accuser - voting on whether to bring specific charges. An impeachment resolution, known as “articles of impeachment,” is like an indictment in a criminal case. A simple majority vote is needed in the House to impeach.

The Senate then conducts a trial. House members act as the prosecutors, with senators as the jurors. The chief justice of the U.S. Supreme Court presides over the trial. A two-thirds majority vote is required in the 100-member Senate to convict and remove a president from office.

No president has ever been removed from office as a direct result of an impeachment and conviction by Congress.

Nixon quit in 1974 rather than face impeachment. Presidents Andrew Johnson in 1868 and Bill Clinton in 1998 were impeached by the House, but both stayed in office after the Senate acquitted them.

Obstruction of justice was one charge against Clinton, who faced allegations of lying under oath about his relationship with White House intern Monica Lewinsky. Obstruction was also included in the articles of impeachment against Nixon.

CAN THE SUPREME COURT OVERTURN?

No.

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

“They quite plainly decided this is a political process and it is ultimately a political judgment,” Bowman said.

“So when Trump suggests there is any judicial remedy for impeachment, he is just wrong.”

PROOF OF WRONGDOING?

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 decide on whatever burden of proof they want,” Bowman said. “There is no agreement on what the burden should be.”

PARTY BREAKDOWN IN CONGRESS?

Right now, there are 235 Democrats, 197 Republicans and three vacancies in the House. As a result, the Democratic majority could vote to impeach Trump without any Republican votes.

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 Democrats. Conviction and removal of a president would requires 67 votes. So that means for Trump to be impeached, at least 20 Republicans and all the Democrats and independents would have to vote against him.

WHO BECOMES PRESIDENT IF TRUMP IS REMOVED?

A Senate conviction removing Trump from office would elevate Vice President Mike Pence to the presidency to fill out Trump’s term, which ends on Jan. 20, 2021.