The draft constitution that Baghdad's political factions have agreed—almost—to submit to the Iraqi people in October addresses grand issues of governance like federalism and Islamism. But that referendum also provides the opportunity to ask Iraqis' consent to a far less abstract question: Whether they want American and allied military forces to stay in Iraq to guarantee security under the new government.

Vowing to "stay in the fight" until "the job is done," President Bush warns against "setting an artificial timetable" for U.S. withdrawal. But a timetable for the departure of foreign troops already exists. The legal authority for the U.S. military to be in Iraq - what distinguishes it from a simple army of conquest—comes from Resolution 1546 of the United Nations Security Council.

That resolution fixed the installation of a government elected under a new Iraqi-written constitution, due by December 31, 2005, as the termination date for the multinational force in Iraq. With the installation of the new government, "the job" the coalition forces were sent to accomplish will be done.

After that date the incoming Iraqi government will have the right to ask foreign troops to help suppress rebellious Iraqis. Perhaps the Bush administration takes it for granted that any new Iraqi government will want American troops on its soil, and apparently the President intends to keep them there for the foreseeable future.

But the people who know best whether Western forces can help, and the people whose lives are most directly affected, are Iraqis themselves. If a majority of Iraqis deems the U.S. military presence essential to their security, their approval in a free election will confound the insurgents, stun al-Jazeera, silence antiwar critics around the world, and endow American troops with the armor of legitimacy they have lacked for two bloody years. Putting the U.S. role to an acid electoral test will be the conclusive proof that America's purpose is democracy, not domination.

The stated cause of the nationalist opposition is the American invaders' immediate departure from Iraq. If the armed insurgents have a chance to achieve that goal by ballots quickly rather than bombings over another decade, they will likely opt to participate in the constitutional referendum rather than boycott it—legitimizing Iraq's democratic process.

Of course, what will entice them is the prospect that the Iraqi public may well vote to send the Americans packing. Polling in Iraq is an inexact science, but public sentiment appears divided enough that a rejection of a continued U.S. military presence is not inconceivable.

With the question of foreign security forces put directly to the people, Iraq's democratic politicians would have to take a public stand on the foreign military presence. Those in today's interim government who know they need American forces will have to say so, publicly and forcefully, to rally their voters. Others may conclude differently: 82 members of the constitutional assembly—nearly a third of the body—have already issued a call for a rapid U.S. pullout and oppose any extension by the U.N. Security Council.

While some officials might be embarrassed if Iraqis chose to give American, British, and Italian troops swift passage home, many Americans would be relieved if Iraqis assume full and immediate responsibility for their own security.

Iraqis will have to weigh what adverse consequences might flow from voting to disinvite American troops, knowing the responsibility for those consequences will be their own. In the best-case scenario, a new Iraqi government unburdened by an occupiers' presence will be able to strike a deal with the nationalist resistance. In the worst-case scenario, an uncompromising resistance might continue to battle a Shiite-dominated regime, pushing the country into civil war.

Yet even in this worst case, Iraqis might rationally calculate that American troops cannot end their civil war—while Arab states might help. The prospect of civil war in Iraq is so unsettling to the Arab world that the phase-out of a U.S. military presence unpopular region-wide can catalyze engagement by the Arab states. They had proposed an Arab-led operation two years ago, before the insurgency gained traction, which Washington scorned. They will have a strong incentive to help Iraqis reach an internal settlement and, by maintaining tranquility, keep Turkey and Iran from intervening.

By relying on the elixir of democracy to determine whether America's war in Iraq ends or continues with the election of a new government, the United States can rehabilitate its scarred reputation internationally and repair the war's bitter polarization of American politics. The administration—or the Congress—should set a referendum as a condition of America's military involvement past December. Either way Iraqis vote, America wins.

Jeffrey Laurenti is a senior fellow at The Century Foundation.