WASHINGTON -- Zimbabwe's Prime Minister Morgan Tsvangirai is attempting something rare and difficult -- sharing power with the man who tried to murder him.

Every Monday morning, Tsvangirai conducts public business across the table from Robert Mugabe, Zimbabwe's president, founder and oppressor. During a recent interview in Washington, Tsvangirai observed to me that the 85-year-old Mugabe "is someone who can be charming when he wants. I am on guard when he becomes charming. It is when I'm most suspicious of his intentions."

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Mugabe has a long history of co-opting his political opponents -- or killing them. "He has not co-opted me," says Tsvangirai. The killing part is not for want of trying. In 1997, regime thugs attempted to throw Tsvangirai out of a 10th-story window. In 2002, he was charged with treason and threatened with a death sentence. In 2007, he was beaten bloody during a protest. And the presidential election that Tsvangirai won last year was clearly stolen by Mugabe.

Yet Tsvangirai is now part of an unlikely power-sharing agreement with Mugabe, becoming prime minister in a unity government. It is, he admits, a "calculated risk."

Tsvangirai describes two calculations. First, he was concerned that Zimbabweans were too weary to take to the streets to contest a stolen election. "You don't want people to reach struggle fatigue. People wanted to try this cohabitation, to ease their economic plight."

Second, Tsvangirai is making the extraordinary calculation that "Mugabe is part of the solution." While most of the rest of the world insists that Mugabe must go, Tsvangirai believes his presence is necessary "to create stability and peace during the transition." The alternative, he fears, could be a destructive militarization of the conflict. And he hopes that the aging Mugabe is considering his own legacy -- choosing to finish his career as the founder of his country, not as the villain of his country.

Given Mugabe's history, this smacks of naivete. But Tsvangirai believes he has a realistic political approach. "You don't expect people who were violent yesterday to wake up one morning and become peaceful." So his strategy is to "build institutions in the course of time" -- particularly through the process of writing a new constitution, leading to new elections. Tsvangirai talks again and again of "institutions" and "mechanisms" and "political architecture" as the methods to make democracy irreversible. His intention is to fight arbitrary and personal rule with the weapons of process -- a Madisonian response to a Neronian dictator.

Four months into the unity government, the results are mixed. The prime minister deserves credit for beginning to stabilize the economy, particularly controlling Zimbabwe's legendary inflation. In August 2008, Zimbabwe's central bank revalued its currency by removing 10 zeroes from its currency; five months later, it removed 12 more. Now the country has essentially scrapped its currency and moved to an economy based on the American dollar and the South African rand. While 70 percent of the population still depends on food aid, goods are back in the stores.

But Mugabe's ruling party remains in charge of the secret police and key ministries. It continues to harass opponents and confiscate farmland. Tsvangirai optimistically calls these elements a "dwindling remnant" -- but it hard to imagine that they will dwindle without a fight. And Mugabe has asserted his dominance with the appointment of political cronies in blatant violation of the power-sharing agreement -- so far with little consequence.

It was this point that Tsvangirai emphasized during his recent U.S. visit, calling on Mugabe's brutal attorney general and corrupt reserve bank governor to step down -- and the world to insist upon these outcomes. This represents a test for South Africa's new president, Jacob Zuma: Will he abandon the "quiet diplomacy" of his predecessor, which often amounted to permission for Mugabe's abuses, and insist that the power-sharing agreement be enforced? It is a test for President Obama: Will he pressure Zuma to do the right thing? And it is a test for the power- sharing agreement itself. A stalemate on these appointments, Tsvangirai admits, would "undermine the credibility of the new dispensation."

Tsvangirai's strategy -- using a power-sharing arrangement with a tyrant to gradually end a tyrant's power -- has little precedent of success. If Tsvangirai fails, he will be just another victim of Mugabe's charming ruthlessness. But if the prime minister succeeds, he will be an exceptional statesman, who set aside his own claims of justice for the peace and progress of his country. And he would become Zimbabwe's true founder.