Senate candidate Roy Moore in Montgomery, Ala., on Sept. 26. (Brynn Anderson/AP)

IT WAS dispiriting that Roy Moore won the Republican nomination to the U.S. Senate the same week that Sen. Bob Corker (R-Tenn.) announced he would be leaving it. But the rot runs deeper than either event would suggest. When Senate leadership is content with the kind of process and dishonesty we witnessed with health-care and now tax "reform," neither event should come as a big surprise.

Certainly Mr. Moore is unfit to be a senator. Over the course of his Alabama primary campaign to fill Attorney General Jeff Sessions's former Senate seat, he pulled out a gun in front of a crowd, falsely claimed that certain Midwestern towns had fallen under sharia law and suggested that "maybe Putin is right" for promoting repressive anti-gay policies. If Mr. Moore were to be seated in the Senate, he would do so after having been removed twice from his position as chief justice of the Alabama Supreme Court — once for his refusal to take down a monument to the Ten Commandments from the courthouse and once for defying the Supreme Court's legalization of same-sex marriage. He has declared that the rule of God must reign supreme to that of law. In 2006, he demanded that Keith Ellison (D-Minn.), newly elected to Congress, be barred from taking federal office because Mr. Ellison is Muslim.

While President Trump campaigned against Mr. Moore at the recommendation of Senator Majority Leader Mitch McConnell (R-Ky.), it would be a mistake to disregard the similarities between Mr. Trump and the insurgent judge. Both have questioned Barack Obama's citizenship and displayed disdain for the rule of law. Like the president, Mr. Moore is scornful of Mr. McConnell and the establishment wing of the Republican Party. He ran on opposition to the Senate majority leader, and his hard-line stances would likely create even more division within an already splintered Republican caucus.

Yet how much moral standing does the establishment wing have to object? Mr. McConnell ended the filibuster for Supreme Court nominations and scuttled normal procedures to push repeal of the Affordable Care Act — leading to Sen. John McCain's (R-Ariz.) call for "regular order," which Mr. Trump mocked on Twitter. Now the Republican establishment is gearing up to pass a tax cut, again likely by dismissing and disparaging the Congressional Budget Office and shunning any Democratic input.

To be sure, the GOP is not the only party guilty of leveraging dysfunction to its advantage: Then-Senate Majority Leader Harry Reid (D-Nev.) chipped away at his share of Senate traditions, too. But if the Republican leadership is afraid of what Mr. Moore could bring to Washington, it may want to reconsider the wisdom of opening the door to his hate-filled appeals and disruptive behavior by backing Mr. Trump in the first place. Otherwise, fewer and fewer public servants in the mold of Mr. Corker, long a voice for responsible governance and bipartisan cooperation, will see any point in coming to Washington in the first place.