WASHINGTON (Reuters) - Investigations involving President Donald Trump by Special Counsel Robert Mueller and U.S. lawmakers have raised the possibility that Congress could seek to remove him from office using the impeachment process set out in the Constitution.

The Constitution assigns different but equally crucial roles to the 435-seat House of Representatives and 100-member Senate.

The House acts as the accuser - voting on whether to bring specific charges - and the Senate then conducts a trial with House members acting as prosecutors and the individual senators serving as jurors. A simple majority vote is needed in the House to impeach. A two-thirds majority is required in the Senate to convict and remove.

Democrats currently control the House. Trump’s fellow Republicans control the Senate.

Mueller is looking into Russia’s role in the 2016 U.S. presidential race, whether Trump’s campaign conspired with Moscow and whether Trump has unlawfully sought to obstruct the probe. Separate investigations are underway in Congress, which also has held hearings including one on Wednesday in which Trump’s former personal lawyer Michael Cohen accused the president of a variety of wrongdoing.

Trump has denied collusion and obstruction. He has called Mueller’s investigation a “witch hunt.” Russia has denied election interference.

FILE PHOTO: The U.S. Capitol building is seen on Capitol Hill in Washington, U.S., January 9, 2019. REUTERS/Jonathan Ernst/File Photo

ON WHAT GROUNDS CAN A PRESIDENT BE IMPEACHED?

Under the Constitution, the president, vice president and “all civil officers of the United States” can be removed from office for “treason, bribery, or other high crimes and misdemeanors,” without being more specific.

Only two U.S. presidents have ever been impeached: Andrew Johnson in 1868 in the tumultuous aftermath of the American Civil War; and Bill Clinton in 1998 over issues including his relationship with a White House intern named Monica Lewinsky. Both times, the House approved formal charges, only to have the Senate fail to convict and remove.

The House Judiciary Committee in 1974 voted to recommend impeachment accusing another president, Richard Nixon, of planning to obstruct an investigation in the Watergate scandal. Before the full House could vote on impeachment, Nixon became the only U.S. president ever to resign.

WHAT IS THE PARTY BREAKDOWN IN CONGRESS?

There currently 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 controlled the House, the chamber voted largely along party lines to impeach Clinton, a Democrat.

Currently, the Senate is made up of 53 Republicans, 45 Democrats and two independents who usually align with Democrats. With conviction and removal of a president requiring 67 votes, that means at least 20 Republicans would have to vote against Trump if all the Democrats and the two independents remained unified in order to remove him from office.

The House Judiciary Committee likely would hold hearings on potential charges against Trump before the full House considers impeachment. If articles of impeachment are approved by the House, U.S. Chief Justice John Roberts would preside over the trial in the Senate, with selected House lawmakers acting as the prosecutors presenting the case on the Senate floor before senators vote on whether to convict.

WHO BECOMES PRESIDENT IF TRUMP IS REMOVED?

A Senate conviction removing Trump from office would automatically elevate Vice President Mike Pence to become president, filling out Trump’s term, which ends on Jan. 20, 2021.

Current U.S. Justice Department policy, first adopted in 1973 and reaffirmed in 2000, opposes the idea of bringing criminal charges against a sitting president, concluding that to do so would violate the separation of powers set out in the Constitution for the three branches of the U.S. government: executive, legislative and judicial.

The Constitution allows that once a president is removed, separate criminal charges could be pursued in court that could result in conviction and punishment potentially including imprisonment.