The October presidential elections in Afghanistan are a watershed moment, equal in importance to the post-Sept. 11 ousting of the Taliban. Now President Hamid Karzai has the chance to embrace -- or to squander -- this moment.

Until a few weeks ago the Bush administration expected the elections to anoint Karzai as the first directly elected Afghan leader, giving him a strong mandate to disarm the warlords, crush a reemerging Taliban and invigorate a long-suffering economy. Karzai's victory also would shine a ray of hope on an otherwise gloomy series of U.S. foreign policy misadventures. But as the campaign began in earnest, a deadly bombing in Kabul signaled the launching of a Taliban effort to disrupt the election. Karzai, meanwhile, finds himself in what could prove to be a competitive race. Even more troubling, in an environment ripe for fraud, his opponents are well placed to manipulate votes and intimidate voters.

How has this happened? The rewriting of the script began with the breakdown of alliances that have buttressed Karzai's fragile rule since 2002. Historically, Afghan politics has degenerated into ethnic blocs; unfortunately, these first democratic elections are proving no exception. In recent opinion polls, many Uzbek, Tajik and Hazara voters support Karzai, a Pashtun from Kandahar. But on Election Day a number of factors may compel Afghans to revert to ethnic labels.

Under strong international pressure, Karzai dropped the most powerful militia commander, Defense Minister Mohammed Fahim, from his ticket. Karzai's flimsy ethnic ties to the Northern Alliance quickly unraveled. Fahim, a Tajik from the Panjshir Valley, aligned with Foreign Minister Abdullah to support the candidacy of a fellow Tajik, Yonus Qanooni, in the presidential race. Recently Ismail Khan, the powerful "emir of the west," was sacked as governor of Herat, chiefly to give Karzai a chance in the province that could have the most voters on Election Day. But as the initial chaos demonstrated, loyalty to Khan is far from being displaced by the central state.

These challenges, along with contenders elsewhere on the ballot -- Gen. Abdurrashid Dostum, an infamous Uzbek warlord; Mohammed Mohaqeq, the Hazara mujaheddin commander, and 14 other candidates -- render Karzai's coronation less than certain. In addition, Taliban attacks in Karzai's core constituency, the Pashtun south, have kept voter registration lower there, and fears of Election Day violence may keep voters from the polls.

In the Uzbek and Tajik north and the Hazara center of the country, there will be little to prevent local militias from stealing votes for their candidates. Local police will guard local polling places, and locally recruited election workers will staff them. Local law enforcement often consists of little more than private militias in public uniforms. The embryonic Afghan National Army, along with international peacekeepers, can do more to guarantee free and fair elections, but these soldiers will be spread thin and in some cases will be held in reserve.

In the face of these fears, the word from Kabul is that the United States is pressuring Karzai to make deals with his most threatening opponents: Bring them, their allies and their armies back into the government in return for their votes. But this would be a short-term fix at the expense of a long-term progress; it would retard any hope for a democratic and peaceful Afghanistan.

The Afghan presidential election of Oct. 9 will present a choice between the old and the new, between a state corrupted by private militias and self-enriching warlords and a new type of government that bases its legitimacy on national rather than ethnic identity. By buying off one or two of the main contenders, Karzai could well guarantee himself a comfortable victory. But he would hamstring his administration and relegate demilitarization, anti-corruption measures and social reforms to the back burner. So far Karzai has resisted the pressure to make deals, but his recent history gives pause. He has never controlled the state through a monopoly of force -- rather, he survived with American backing and by cutting deals with the biggest guns.

Where to go from here? Real progress will come only through taking on the old-style power brokers in an election and beating them back. Other nations can help not only by encouraging Karzai to hold firm but also by paying attention to the election process itself. In a reasonably fair election, Karzai is likely to win; he has the broadest multi-ethnic appeal and represents a significant break with the past. He may not win a first-round victory against 17 other candidates, but in a subsequent runoff, the opposition will fragment and the choice between old and new will come into sharper focus.

And yet, at present, there are hardly any international election monitors scheduled to watch the vote, and domestic election observers lack resources. Security undoubtedly hinders election observation, but if the Organization for Security and Cooperation in Europe, the European Union and U.S. organizations are unwilling to send significant teams, how can Afghans be assured of a fair vote? A legitimately elected administration in Kabul would not just be good for the Afghans; it would be much more likely to carry out the reforms the United States so keenly wants.

The writer, a political science professor at the University of North Carolina at Chapel Hill, recently returned from an election assessment mission in Afghanistan for the Afghanistan Research and Evaluation Unit, an independent research organization.