A test is shaping up for one of the Constitution’s most adamant provisions, the Speech or Debate Clause, now that Representative Nancy Mace is being sued for a speech she delivered on the House floor. In February, Ms. Mace herself characterized her tirade as a “scorched earth” speech. She accused four men in her district, including her ex-fiancé, of “some of the most heinous crimes against women imaginable.” One of them is now suing her for defamation.