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.