ZIMBABWE, which for more than 30 years has suffered under the arbitrary and autocratic rule of Robert Mugabe, may have a chance to free itself through a democratic vote. On March 16 more than 3 million of the country’s voters turned out to approve a new constitution, opening the way — years later than promised — for a new presidential election. The vote, which could take place this summer, will probably pit the 89-year-old Mr. Mugabe against his longtime rival Morgan Tsvangirai, who likely would win a free and fair ballot.

That’s where the likely trouble starts. Mr. Tsvangirai’s victory in the first round of the last presidential election, in 2008, prompted an orgy of violence by the Mugabe regime, which assaulted villages that supported the opposition and arrested and tortured many of its leaders. Some 200 people were killed; Mr. Tsvangirai refused to participate in a runoff election and Mr. Mugabe claimed a new mandate. Only the opposition leader’s subsequent agreement to join the government as prime minister stopped a catastrophic economic meltdown.

With the U.S. dollar as its currency, the Zimbabwean economy is now growing fast, thanks in part to heavy government spending backed by loans from China. The new constitution, though flawed in some important respects, contains guarantees of human rights. But there are signs that the regime is returning to the same tactics that kept Mr. Mugabe in power four years ago. The day after the constitutional referendum three senior members of Mr. Tsvangirai’s Movement for Democratic Change were arrested, along with a leading human rights lawyer who later was released on bail.

That followed a series of raids against civic groups and grass-roots activists, like the Zimbabwe Peace Project, which investigates human rights cases. According to a report by the BBC, police in rural areas have begun confiscating radios capable of receiving foreign broadcasts — presumably so that there will be no competition for the state broadcaster during the upcoming campaign.

The only way to stop Mr. Mugabe and the thugs around him from manipulating the coming election is aggressive intervention by outsiders. That starts with Zimbabwe’s neighbors in the Southern African Development Community, which permitted the regime to get away with the outrages of 2008; South Africa and other governments must stand up against the incipient abuses before it is too late.

Western governments also have an important role to play. Following the constitutional referendum, the European Union suspended sanctions against Zimbabwean companies and most regime figures other than Mr. Mugabe and a handful of cronies; that was premature. The Obama administration has more wisely held off, with a State Department spokesman suggesting that “what we need is to see this serve as a precedent for upcoming presidential elections.” Unless and until Zimbabweans are given a fair and free chance to choose between Mr. Mugabe and his challengers, U.S. sanctions should remain in place.