Unless you’re a historical trivia buff, you probably don’t remember these episodes. But most politically aware Americans can name the two presidents who were impeached — Andrew Johnson and Bill Clinton — and the one who resigned to avoid that fate, Richard Nixon. That ignominious distinction is central to all three presidencies. And although Johnson and Clinton avoided removal by the Senate, their legacies were permanently scarred. Even if it fails to produce a conviction in the Senate, impeachment by the House is censure with teeth.