Abigail Spanberger won a landslide victory in the Virginia governor’s race last November with a platform that focused on reining in rising electricity costs . Virginia is home to the world’s largest concentration of artificial-intelligence data centers, and the state’s biggest utility is straining to meet an expected surge in power demand. Spanberger, a Democrat, promised on the campaign trail to “make Virginians’ bills more affordable.”

It might seem surprising, then, that the new governor signed a bill last month that would return Virginia to the Regional Greenhouse Gas Initiative, or RGGI, a carbon pricing program that covers electrical utilities in states across the Northeast and mid-Atlantic.…