(Reuters) - The U.S. House of Representatives is conducting an impeachment investigation of President Donald Trump following complaints about a July 25 phone call in which he pressed Ukraine’s president to investigate Joe Biden, a leading contender for the Democratic nomination in 2020.

FILE PHOTO: An executive order sits atop a desk adorned with the presidential seal prior to U.S. President Donald Trump signing executive orders on "transparency in federal guidance and enforcement" in the Roosevelt Room of the White House in Washington, U.S., October 9, 2019. REUTERS/Jonathan Ernst

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 in the Constitution a process for removing a president.

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.

The president, under the Constitution, 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, including trying to obstruct judicial proceedings.

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

HOW DOES IT WORK?

Impeachment begins in the House, the lower chamber, 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.

A trio of House committee are currently interviewing witnesses and issuing subpoenas for documents to build a case against Trump — what they have called an “impeachment inquiry.”

The White House and congressional Republicans have called this process illegitimate, saying that the House should have voted as a body on whether to authorize the impeachment investigation.

Such full House votes were held in the Clinton and Nixon impeachments. But constitutional lawyers said a full House vote is not required because the Constitution gives the House broad discretion to conduct the impeachment process as it sees fit.

If the House approves articles of impeachment, a trial is then held in the Senate. 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 the 100-member Senate to convict and remove a president. This has never happened.

CAN THE SUPREME COURT OVERTURN?

No. 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 allowing an appeal of a Senate conviction to the federal judiciary.

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

WHO BECOMES PRESIDENT IF TRUMP IS REMOVED?

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.