There have only been three times in American History that a Presidential impeachment process has been initiated by Congress:
1, Andrew Johnson, Democrat/National Union, was impeached on February 24, 1868, by the House of Representatives after violating the newly created Tenure of Office Act by a 126 to 47 vote. President Johnson was acquitted by the Senate, which voted 35–19 in favor of conviction, falling one vote short of the necessary two-thirds needed to remove him from office. The Tenure of Office Act would later be found unconstitutional by the Supreme Court of the United States in dicta.
2. Impeachment proceedings against Richard Nixon were referred to the full House of Representatives for consideration and ended with his resignation.
3. Bill Clinton, Democrat, was impeached on December 19, 1998, by the House of Representatives on articles charging perjury (specifically, lying to a federal grand jury) by a 228–206 vote and obstruction of justice by a 221–212 vote. The House rejected other articles: one was a count of perjury in a civil deposition in Paula Jones' sexual harassment lawsuit against Clinton (by a 205–229 vote), the second accused Clinton of abuse of power (by a 148–285 vote). President Clinton was acquitted by the Senate. The votes to remove him from office fell short of the necessary two-thirds: 45–55 on obstruction of justice and 50–50 on perjury.
It is important to keep in mind that no U.S. President has ever had an impeachment conviction in the U.S. Senate in American history. In other words, the entire impeachment and conviction process, whereby the House votes to impeach a President by a simple majority, and then the Senate votes to convict a President by a two-thirds majority, has never happened. Former Presidents Andrew Johnson and Bill Clinton were impeached in the U.S. House of Representatives, but neither was convicted in the Senate. In the case of former President Richard Nixon, impeachment proceedings had begun when he resigned from office in 1974, but a full U.S. House vote on impeachment had not taken place when he chose to resign from office. The first article of impeachment against Nixon--obstruction of justice—had been drawn up, and a House subcommittee had voted 27-11 to bring the first article of impeachment to the full U.S. House for a vote, but the full U.S. House had not yet voted on the first article of impeachment when Nixon resigned. Former President Richard Nixon was, at the time he resigned, a single House vote away from being impeached.
If one believes that there might be a need to rid the country of a President, and one is interested in using the impeachment process to do this, one should pay special attention to the lessons learned from both the Clinton Investigation and impeachment and the Nixon investigation and attempt at impeachment.
The Nixon Investigation and Initial Impeachment Proceedings
The key thing to note from the Nixon situation is that it was the Senate investigation and the corresponding open Senate hearings that fried Nixon. By the time the U.S. House began its initial impeachment proceedings against Nixon, not only had all of the Senate Democrats decided that Nixon needed to go, but enough Republicans in the Senate had also decided that he needed to go that Republicans knew that Nixon would be convicted in the Senate after he was impeached in the House. So, three ranking Republicans visited Nixon at night and told him that he needed to resign before he was impeached and convicted. Nixon announced on television the next day his intention to resign. Again, it was the very thorough Senate investigation and very thorough open Senate hearings that basically sunk Nixon before the U.S. House had even voted to impeach Nixon.
The Clinton Investigation and Impeachment Proceedings
Republicans were so anxious to impeach former President Bill Clinton that they ignored what they should have learned from the Nixon situation--that the key to getting rid of a President is to have thorough and open hearings before one even gets to the impeachment process so that the President's goose is already cooked before impeachment proceedings begin. Republicans rushed into impeaching Clinton soon after Kenneth Star published his report—without any open Senate hearings at all. America was doing well at the time, including the American economy, and most Democrats in Congress thought that the charges against Clinton were contrived. Without a sufficient number of Democrats willing to convict Clinton in the Senate, Clinton was not convicted and never was going to be convicted.
In addition, the American public was so annoyed at the complete waste of time of the Clinton impeachment process that the Democrats actually gained seats during a midterm while having a Democrat as President, only one of two times in modern history that this has ever happened (the two times in modern history occurred in 1998 after Republican's failed attempt to convict Clinton and in 2002, post 9/11, under George W. Bush). The lesson here is if you do not make sure to fry a President in open hearings before going to impeachment proceedings, the political party impeaching can both fail to convict and also experience a backlash at the next election. Republicans thought that the impeachment hearings involving President Bill Clinton would help them in the 1998 midterms, but their overreach created a backlash, and they lost seats in Congress instead.
Conclusion
The Nixon and Clinton situation, as well as the failed Andrew Johnson impeachment proceedings (in which Johnson was impeached by the House but the Senate failed to convict—by one vote) all tell us the same thing—that the way to get rid of a U.S. President is to have a thorough investigation and complete open session hearings before the start of impeachment proceedings to give the American public, and Senators from the opposing party, time to absorb the wrongs committed by that President. Not to do that risks wasting everyone’s time, risks failing to rid the country of an unwanted and perhaps criminal President, and risks creating a backlash against the party starting the impeachment process.
This is why Nancy Pelosi wants to have all of our ducks in a row before we begin any impeachment. I am sure Nancy Pelosi knows well both the impeachment history of Nixon and Clinton. She does her homework. There are a ton of Trump administration and campaign officials in the past two years who gave testimony behind closed doors while Republicans were in charge of Congress who were allowed not to answer those questions that they did not want to answer. By collecting information from the upcoming Mueller Report, by collecting information from current U.S. House investigations, and by collecting information gathered by reexamining witnesses, improperly interviewed by Congressional Republicans, we should be able to build a mountain of evidence against Donald Trump before we begin impeachment proceedings.
History has taught us that this is exactly the method that Congressional Democrats should use to prepare for any possible impeachment. This is what Nancy Pelosi realizes. She is not saying no to impeachment. Instead, she is saying there is no advantage to launching impeachment proceedings before all of the evidence has been collected and laid before both Republicans and the entire American Public. With proper preparation, and perhaps a pinch of luck, the Republicans may soon once again be notifying a Republican President of the need to resign even before initial impeachment proceedings are finished. I believe that I could learn to live with the news that Mitch McConnell had made a trip with his Senate buddies in the cover of night to tell Trump to get out, just like it happened with Nixon. I would miss out on impeachment, but I would persevere. :)