MARK HERRING (D), Virginia’s brand-new attorney general, has decided to participate in a legal attack against a provision of the state’s own constitution, joining two couples in federal court to argue that Virginia’s ban on same-sex marriage violates the U.S. Constitution. GOP critics allege that Mr. Herring is disregarding his obligation to defend Virginia’s laws, even turning the state into a “dictatorship” in the process. The National Organization for Marriage wants him impeached. That sort of outrage is not even close to called for.

We broadly agree with Mr. Herring’s reading of the law. The Constitution’s guarantee of equal protection cannot be reconciled with denying, on logically flimsy grounds, equal access to civil marriage for a group that has for centuries been singled out for discrimination.

Yet the instinct, tradition and expected practice of the government’s legal representatives must be to defend duly established laws against legal challenge, even those with which they personally disagree. During the George W. Bush administration, it was noble and proper of then-Solicitor General Theodore Olson, a strong conservative, to defend federal campaign finance laws against legal attack. Only when attorneys general are convinced that no reasonable argument could vindicate a law under challenge should they feel comfortable doing anything but aggressively defending enacted statute.

The reason Mr. Herring’s move isn’t wholly unwise, and shouldn’t be used to excuse less-
responsible deviations from the norm, is that this issue is highly unusual. The constitutional case for equal marriage isn’t just strong; the ultimate arbiters of what is and is not constitutional are well on their way to saying so. In rulings last year, the Supreme Court pointed toward eventually declaring equal marriage rights to be a constitutional guarantee. Two federal district judges have already taken the justices’ lead and overturned same-sex marriage bans in other states. The worst that can be said of Mr. Herring and officials doing similar things in other states is that they are running ahead of the Supreme Court. That’s a stretch of their official responsibilities, but it’s hardly a rank betrayal of duty.

It would have been unacceptable, too, if Mr. Herring hadn’t made sure that someone would defend Virginia’s ban in court. But the defendants in the suit, two county clerks, have attorneys, one paid for by a state fund, the other by a national activist organization. Members of the General Assembly are also examining ways to hire counsel to represent them before the court.

We worry that Mr. Herring’s decision will add to the nasty partisanship that increasingly grips Richmond. It won’t be lost on anyone that the attorney general’s move stands to set him up as a leading contender for the next Democratic gubernatorial primary. Instead of allowing this to further polarize the capital, Republicans should tone down their outrage. And Mr. Herring should make clear that this is an extraordinary circumstance and that he will live up to the competence and professionalism that Virginians traditionally have expected of their leaders.