Imagine if Monica Lewinsky had taken the stand as a witness in President Clinton’s 1999 impeachment trial. Imagine the drama of the young former White House intern sitting in the well of the Senate recalling how the president encouraged her to sign a false affidavit after learning that she would be a witness against him. It would have been an unforgettable moment. But it didn’t happen for the simple reason that few in the Senate wanted to hear such evidence.

During the Clinton trial, the House impeachment managers were surprised...