RE: Nixon: “I am not a crook.” — Impeached for covering up a political break in.
Nixon was not impeached. He resigned rather than subject himself or the entire nation to the spectacle of an impeachment proceeding.
Clinton on the other hand...
Unfortunately, Clinton wasn't inpeached either. The Senate refused to act on the Articles of Impeachment prepared by the House.
Remember all the Senate RATS gathering outside on the Senate steps in their overcoats in a rally for Clinton?