SCENARIOS - Possible outcomes of Afghan election

Afghans walk past election billboard of Afghanistan's President Hamid Karzai in Herat, western Afghanistan August 18, 2009, file photo.

KABUL (Reuters) - Polls predict Afghan leader Hamid Karzai will win Thursday's presidential election, but not with the outright majority needed to avoid a run-off against his chief challenger, former foreign minister Abdullah Abdullah.

Taliban fighters have vowed to disrupt the poll, and violence could increase the chance of a run-off by suppressing turnout in the south, where Karzai draws much of his support.

Two separate polls, commissioned by the U.S. government and published last week, gave Karzai about 45 percent of the vote and Abdullah about 25 percent.

Here are some scenarios that may unfold:

- KARZAI FACES A RUN-OFF AGAINST ABDULLAH

The president would still be the front runner, but a two-horse race could be a challenge because his opponents would finally have a standard-bearer to rally around.

Karzai relies in part on the support of powerful former guerrilla chiefs and regional bosses. Some could peel away if they are no longer certain he will win, while others may make more forceful demands for a future role in his government.

Abdullah's roots are in a northern, mainly ethnic Tajik anti-Taliban guerrilla movement, and he still draws most of his support from the north, making it difficult for him to win enough southern support to defeat Karzai in a second round. But his father was a Pashtun from the south, and Abdullah seems to have had at least some success in broadening his support beyond his movement's traditional base.

A second round would mean at least six more weeks of uncertainty and provide the Taliban with another high-profile event to target. But it could also be presented as proof that the first round was not fixed, and therefore help convince Afghans that the democratic process is genuine.

- KARZAI WINS IN FIRST ROUND

Karzai remains the only candidate with a serious shot at winning more than 50 percent of the vote in the first round. He won 55 percent in the country's first democratic election in 2004, and some of those who ran against him then, like Uzbek former militia leader Abdul Rashid Dostum, support him now.

Afghans have mixed views about Karzai. Security in parts of Afghanistan has deteriorated badly since 2004 despite an influx of foreign troops, and many Afghans complain that Karzai's government is corrupt, ineffective and beholden to the West.

Even as war intensifies in some areas, however, much of the country is at peace for the first time in decades and desperate poverty is easing. Despite discontent with his government, Karzai himself remains personally popular: in one poll, 81 percent of respondents had a favourable view of him and only 17 percent had an unfavourable view, consistent with other surveys. Abdullah is also well-liked, with a favourable rating of 71 percent.

A declaration of a first-round victory could raise the chance of civil unrest if the opposition does not accept the result, although Abdullah has played down suggestions his followers would take to the streets.

Speculation would quickly turn to the composition of a future Karzai government, after he relied so closely on former militia leaders for election support. Diplomats say his government now includes capable technocrats, but they worry that these officials could be replaced by former warlords who were promised jobs.

- VOTING DISRUPTED BY VIOLENCE

Violence this year is at its worst since the fall of the Taliban in 2001, and has worsened ahead of the poll. The Taliban have called for a boycott and have vowed to disrupt the vote.

Over the past week their attacks have focused on Kabul, the capital, which had been comparatively secure for months.

The government has ordered journalists not to report violent incidents while polls are open, an effort it says is to keep Afghans from being frightened away from the polls.

A U.N. report has said intimidation and attacks have already interfered with election preparations and campaigning, and could prevent many Afghans from voting. U.S. officials think attacks are unlikely to reach a level that would scupper the poll.

The violence is mainly concentrated in the south, which is where Karzai draws much of his support. Violence that suppresses turnout there could therefore increase the chance of a run-off. Violence also hinders efforts to prevent fraud, which could add to doubts about the legitimacy of the result.

There are more than 100,000 Western troops in the country, promising to impose an outer perimeter of security, while Afghan troops and police will handle security in towns and villages.

- VOTING MARRED BY FRAUD

Western diplomats say their main goal is to ensure that the election is seen as credible and legitimate but huge distances, powerful local chiefs, low literacy rates and weak institutions make it difficult to prevent abuse.

There have already been reports of fake voter registration cards for sale and of suspiciously large numbers of women being registered -- possibly by men who could try to cast votes on their behalf, a practice that could allow fraud. (For more Reuters coverage of Afghanistan and Pakistan, see: here)