Maine Governor Janet Mills’s decision to suspend her U.S. Senate campaign and effectively concede to upstart Graham Platner is a huge temporary victory for progressive Democrats in their ongoing fight with the party’s center-left establishment wing. But it will only be a true victory for progressives if Planter wins in November. And the left will be diminished if the untested Platner loses the general election because of baggage from his past or political inexperience—the unknowns that left