See? There really is the occasional difference when Democrats are elected:

Virginia Attorney General Mark R. Herring will announce Thursday that he believes the state’s ban on same-sex marriage is unconstitutional and that Virginia will join two same-sex couples in asking a federal court to strike it down, according to an official close to the attorney general with knowledge about the decision.

The action will mark a stunning reversal in the state’s legal position on same-sex marriage and is a result of November elections in which Democrats swept the state’s top offices. Herring’s predecessor, Republican Ken Cuccinelli II, adamantly opposes gay marriage and had vowed to defend Virginia’s constitutional amendment banning such unions, which was passed in 2006 with the support of 57 percent of voters.

Herring, too, had voted against same-sex marriage eight years ago, when he was a state senator. But he has said that his views have changed since then and that on Thursday he will file a supportive brief in a lawsuit in Norfolk that challenges the state’s ban, said two people familiar with his plans.

Herring will say that Virginia has been on the “wrong side” of landmark legal battles involving school desegregation, interracial marriage and single-sex education at the Virginia Military Institute, one official said. He will make the case that the commonwealth should be on the “right side of the law and history” in the battle over same-sex marriage.

He has not informed Republicans in Richmond about his plans; an uproar is likely. GOP lawmakers have worried that Herring would change the state’s position — such decisions are up to the attorney general — and have contemplated legislation that would allow them to defend the law in court.

The attorney general thinks that is unnecessary, the official said. The clerks of the circuit court in Norfolk and Prince William County are defendants in the suit, and both are represented by independent counsel.

Janet Rainey, the state registrar of vital records, is also a defendant. Although she and Herring will urge the court to strike down the ban, she will continue to enforce it until the courts act.