Sept 24 (Reuters) - The U.S. House of Representatives on Tuesday launched an official impeachment investigation of President Donald Trump after he encouraged a foreign leader to conduct a probe that could damage a political rival.

House Speaker Nancy Pelosi, a Democrat, announced the investigation at a news conference, declaring “no one is above the law.”

There has been a groundswell of support among Democratic Party lawmakers for the move following Trump’s public admission that he asked Ukraine President Volodymyr Zelenskiy to investigate the son of presidential candidate and former Vice President Joe Biden.

Such an important day at the United Nations, so much work and so much success, and the Democrats purposely had to ruin and demean it with more breaking news Witch Hunt garbage. So bad for our Country! — Donald J. Trump (@realDonaldTrump) September 24, 2019

Trump wrote on Twitter on Tuesday that the July 25 phone call was “very friendly and totally appropriate” and that he put”no pressure” on Zelenskiy. He later called the House probe”Witch Hunt Garbage” in a tweet.

The following explains how the impeachment process works.

WHY IMPEACHMENT?

The founders of the United States created the office of the presidency and feared its powers could be abused. So they included in the U.S. Constitution a procedure for removing a sitting president from office.

Under the Constitution, the president can be removed from office for “treason, bribery, or other high crimes and misdemeanors.”

What exactly that means is unclear. Historically, it can encompass corruption and other abuses of the public’s trust.

A president does not need to have violated a specific criminal law to have committed an impeachable offense.

Many legal commentators have said that pressuring a foreign leader to interfere in a U.S. election is the sort of conduct the nation’s founders would have considered an impeachable offense.

HOW DOES IT WORK?

A misconception about “impeachment” is that it refers to the removal of a president from office. In fact, impeachment refers only to the House, the lower chamber of Congress, bringing charges - similar to an indictment in a criminal case.

There is ongoing debate over how an impeachment investigation should begin. Doug Collins, the leading Republican on the Judiciary Committee, has argued that a formal impeachment investigation does not begin until the full House has voted to authorize it. But Democratic lawmakers have argued that such a vote is not necessary.

The House Judiciary Committee has historically led impeachment investigations, but Democratic Party leaders can also opt to put a select, handpicked committee in charge.

If a simple majority of the House’s 435 members approves bringing charges, known as “articles of impeachment,” the process moves to the Senate, the upper chamber, which holds atrial to determine the president’s guilt.

In such a trial, House members act as the prosecutors, the senators as jurors; the chief justice of the U.S. Supreme Court presides. A two-thirds majority vote is required in the100-member Senate to convict and remove a president.

Lawmakers are not required to prove their case beyond a reasonable doubt — the evidentiary standard in a criminal case.

PARTY BREAKDOWN IN CONGRESS?

The House has 235 Democrats, 199 Republicans, and one independent. 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 President BillClinton, a Democrat. Two and a half months passed between theHouse voting to move forward with impeachment proceedings against Clinton and his impeachment.

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 via impeachment, at least 20 Republicans and all the Democrats and independents would have to vote against him.

The Republican majority in the Senate could vote to immediately dismiss the charges against Trump without considering evidence.

No president has ever been removed as a direct result of impeachment. One, President Richard Nixon, resigned in 1974before he could be impeached. Two, Presidents Andrew Johnson in1868 and Clinton, were impeached by the House, but not convicted by the Senate.

WHO BECOMES PRESIDENT IF TRUMP IS REMOVED?

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

IS THERE ANOTHER WAY TO REMOVE A PRESIDENT?

Under the Constitution’s 25th Amendment, a president can be replaced by their vice president if the chief executive becomes unable to do the job, such as due to a disabling medical or mental condition. That process begins with the vice president and a majority of the members of the Cabinet notifying Congress that the president is not capable of performing the job.