Rep. Morris Udall, on losing the 1976 presidential nomination to Jimmy Carter, quipped: “The voters have spoken—the bastards.” That applies to Alabama’s Dec. 12 U.S. Senate election. The voters will have their say, and if they elect Roy Moore despite his alleged sexual misconduct, the Constitution requires the Senate to seat him, and expelling him would violate congressional precedent.

Senate Republicans have reportedly discussed refusing to seat Mr. Moore, but legally they cannot. In 1967 the House excluded Rep.-elect Adam...