In ruling that a justice on West Virginia’s highest court should have recused himself from a case involving a campaign benefactor, the U.S. Supreme Court has righted an egregious wrong. More important, the 5-4 decision will encourage judges to avoid both the appearance and the reality of conflicts of interest.

Monday’s decision involved a nightmare scenario for those who believe that courts should be impartial. In 2002, a jury concluded that the A.T. Massey Coal Co. had driven a competitor into bankruptcy by engaging in fraudulent business practices, and it ordered Massey to pay $50 million in damages. Don L. Blankenship, Massey’s chief executive, then spent $3 million to oppose the re-election of Warren McGraw, a justice on the West Virginia Supreme Court of Appeals.

McGraw was defeated in 2004, and the man who took his seat was Brent Benjamin. When Massey’s appeal reached the state’s high court, Benjamin was part of a 3-2 decision overturning the verdict against the company. Two other justices had recused themselves, one because he had criticized Blankenship’s involvement in the 2004 election, the other because he had vacationed with Blankenship on the French Riviera. But Benjamin refused to withdraw despite his obvious obligation to Massey’s CEO — though Blankenship had only given $1,000 to Benjamin’s campaign, he spent enormous sums on separate efforts to defeat McGraw.

Benjamin’s refusal, Justice Anthony Kennedy wrote for the majority, violated the due process clause of the 14th Amendment because “the probability of actual bias on the part of the judge or decision maker is too high to be constitutionally tolerable.”

That language comes from a 1975 Supreme Court decision involving the suspension of a medical license. In applying it to the relationship between a judge and a campaign benefactor, the court is breaking new ground. But Kennedy’s “probability of actual bias” test would seem to apply not only to elections but to other potential conflicts — for example, when a sitting judge is wined, dined or lavished with gifts by a litigant.

Oddly, Kennedy’s opinion minimizes its own importance, stressing that its rule would “be confined to rare instances.” Chief Justice John Roberts Jr., writing for the dissenters, had the better of this argument when he wrote that Kennedy’s claim of rare application of the new rule is “so much whistling past the graveyard.” Roberts is right that the new test will sweep widely. He’s wrong to bewail a decision that will force judges, including members of his own court, to take apparent conflicts of interest more seriously.