Threats of impeachment of a U.S. President have been historically rare. At least three conditions must be met for it to be in play as a political topic: There must be a party imbalance with a President of one party and a majority in the House of the opposite party, as well as at least sympathy or a split opinion in the Senate; there must be a plausible "High Crime or Misdemeanor" to generate the charges; and there must be enough time for the process to carry through before an election changes the map once again.
Out of the list of election cycles where the party imbalance existed, the other two conditions were met only three times in American History.
The Impeachment Clause in the U. S. Constitution
The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors.
Article III Section 4. Removal
Andrew Johnson
Texts
Electronic Resources
History of the Impeachment of Andrew Johnson. Avalon Project. Lillian Goldman Law Library, Yale Law School.
Impeachment_of_Andrew_Johnson (wiki)
References
Richard M. Nixon
Texts
Electronic Resources
References
William J. Clinton
Texts
Bumpers, Dale. Defense in the Clinton Impeachment Trial.
Clinton, William J. "I Have Sinned"
Clinton, William J. State of the Union, 1999.
Hyde, Henry. Summation of the Prosecution in the Clinton Impeachment Trial.
Donald J. Trump
Texts
Trump, Donald. Donald Trump, Message after Pro-Trump Mob Overruns Capitol, 07 January 2021.
Electronic Resources
References
January_6_United_States_Capitol_attack
United_States_House_Select_Committee_on_the_January_6_Attack
First_impeachment_of_Donald_Trump