President Richard Nixon was not impeached. On July 27, 1974, the House Judiciary Committee recommended the impeachment and removal from office of president for obstructing justice, abuse of power and contempt of Congress. Before the required vote, Nixon resigned, effective Aug. 9, 1974.
Impeachment starts with a resolution by a member of the House of Representatives. The House Judiciary Committee decides whether the allegations stated in the resolution merit further action, subject to House approval. When the investigation shows evidence of wrongdoing, the Committee refers the matter back to a vote of the full House of Representatives. Specific charges are considered separately. A simple majority vote on any one charge impeaches the president at this point. The House can then send the articles of impeachment to the Senate for trial. Richard Nixon resigned before the House of Representatives could vote for impeachment.