The process for doing so is laid out simply in the Constitution, if anyone bothers to read it.
- Someone in the House must introduce a bill of impeachment, laying out the crimes and misdemeanors the President is guilty of.
- The bill will be referred to the House Committee on the Judiciary, where a majority of the committee (in this case, staffed at a 3-to-2 ratio by Republicans) must approve of the bill. This means the chairman of the committee must approve of holding hearings, hearings must be held, and the bill marked up (e.g., changes made). Then a vote is held.
- The bill then goes to the House Rules Committee, which assigns it to a "calendar" for legislation action. The Rule Committee chairman must approve of holding hearings, hearings must be held, and the rules for debate, amendments, and other aspects of the legislative process approved. These rules must win a majority.
- The bill then waits on the calendar for its turn to come up. Debate and amenments and votes are held under the rule approved by the Rules Committee.
- The House may bypass the Judiciary and Rules committees if a majority of House members sign a petition asking for the bill to be discharged to the floor.
- The House will sit as a Committee of the Whole, taking testimony from witnesses and receiving documentary evidence. A simple majority is required to send the bill of impeachment to the full House.
- The House, sitting as the House, will then vote to impeach. A simple majority is needed to impeach.
- The Senate is then required to hold a trial, presided over by the Chief Justice of the Supreme Court. Senate rules of procedure govern the timing of the trial, its conduct, the number of witnesses allowed, and the amount of debate permitted.
- A two-thirds vote (67) of all sitting Senators is required for impeachment.
But it's almost impossible to get the party of the President to impeach him, even when there is outstanding evidence.
Let's look at Watergate, shall we?
When Richard Nixon was impeached by the House for undermining democracy, illegal wiretapping, conspiracy, breaking-and-entereing, destruction of documents, cover-up, bribery, obstruction of justice, and various misdemeanors.... Republicans in the House could barely bring themselves to do it.
Democrats were in overwhelming control of the House. 19 resolutions had been offered (all by Democrats) asking the Judiciary Committee to hold hearings on whether grounds existed to impeach the President; three resolutions had been introduced censuring the President; and three resolutions had been introduced to "investigate the official conduct of the President". On top of this, there were 20 bills of impeachment which had been offered. Half of them came from three Californians (mostly from Jerome Waldie and Bob Leggett). Seven more came from liberal eastern states (New York , New Jersey , and Maryland ). All came from Democrats, none from Republicans.
The House Judiciary Committee investigated Watergate for 18 months without considering a single bill of impeachment -- although more than 20 had been filed. It took the "Saturday Night Massacre" (in which Nixon fired Watergate Special Prosecutor Archibald Cox, Attorney General Elliot Richardson, and Deputy Attorney General William Ruckelshaus) to get the Judiciary Committee to begin taking up these bills of impeachment. No Republican voted to allow the committee to do so; these were straight 21-to-17 party line votes.
Judiciary Committee chairman Peter Rodino was so worried about the partisan look of things, he asked the entire House for authority to hold hearings on impeachment. Minority Leader John Rhodes (R-Ariz.) broke on February 2, and endorsed the resolution. On Febraury 6, the House voted 410-4 to give Rodino's committee the explicit power to hold impeachment hearings.
The vote was seen as political cover for Republicans. Six special elections had been held to fill seats in the House where members had died or resigned. Five safe GOP seats went Democratic. No one believed that there were more than 100 votes in the House for impeachment, and impeachment required 218.
On March 1, a grand jury sitting in the District of Columbia named Nixon an unindicted co-conspirator in the Watergate scandal.
Since July 16, Democrats and prosecutors had been struggling to get hold of the Nixon tapes. The Judiciary Committee finally voted 33-to-3 on April 11, 1974, to subpoena them. Voting against were ranking minority member Edward Hutchinson (R-Mich.), Trent Lott (R-Miss.), and Charles Wiggins (R-Calif.) (Republicans Charles Sandman [R-N.J.] and Harold Froelich [R-Wisc.] did not attend the meeting and so did not vote.) Once more, the vote was political cover: The Democrats were united and controlled a majority on the committee. They'd come to the hearing with a subpoena already drafted. The GOP could either go along, or not. Electoral safety dictated going along. Republicans offered several amendments in committee, some of which asked Nixon to voluntarily turn over the tapes, some of which asked Nixon only for transcripts, and so on. All were defeated by an almost-party line vote of 24-to-12 (only three Republicans voting against them).
Nixon provided only edited transcripts, which the Judiciary Committee rejected. Impeachment hearings opened on May 9.
Committee votes on the actual articles of impeachment occurred July 27-30. They passed on almost strict party lines: Article 1 (obstruction of justice), 27–11; Article 2 (abuse of power), 28–10; Article 3 (contempt of Congress), 21–17 (two Democrats voted no). Article 4 (Cambodia bombing) failed 12–26, and Article 5 (failure to pay taxes) failed 12–26.
On July 24, the Supreme Court ruled unanimously Nixon must release the actual tapes, not just edited transcripts. On August 5, 1974, the White House released a previously unknown audio tape from June 23, 1972. Recorded only a few days after the break-in, it revealed Nixon not only knew in advance about plans for a break-in, but collaborated actively in the cover-up, working to block the law enforcement investigation and bribe the burglars to keep their mouths shut. The "smoking gun tape" destroyed Nixon politically. The 10 Republican members of the House Judiciary Committee who had overwhelmingly supported Nixon now announced they would all support the obstruction of justice article.
On the night of August 7, 1974, Sens. Barry Goldwater and Hugh Scott and Rep. John Rhodes met secretly with Nixon and told him that his support in Congress had all but disappeared. Support for impeachment in the House was way over the 218 needed, and no more than 15 senators would support him (he needed 34).
With impeachment all but certain, Nixon resigned on the night of August 8, 1974.