The sweeping order, in a swing state that could play a role in deciding the November presidential election, will enable all felons who have served their prison time and finished parole to register to vote. Most are African-Americans, a core constituency of Democrats, Mr. McAuliffe’s political party.

“There’s no question that we’ve had a horrible history in voting rights as relates to African-Americans — we should remedy it,” Mr. McAuliffe said Thursday, previewing the announcement he will make on the steps of Virginia’s Capitol, just yards from where President Abraham Lincoln once addressed freed slaves. “We should do it as soon as we possibly can.”

The action, which Mr. McAuliffe said was justified under an expansive legal interpretation of his executive clemency authority, goes far beyond what other governors have done, experts say, and will almost certainly provoke a backlash from Virginia Republicans, who have resisted measures to expand felons’ voting rights. It has been planned in secrecy, and comes amid an intensifying national debate over race, mass incarceration and the criminal justice system.