Q+A: What are the stakes for U.S. in Afghanistan vote?

WASHINGTON (Reuters) - Afghans were set to vote in a presidential election on Thursday under threat of violent disruption by the Taliban in a critical test of President Barack Obama's strategy in what he calls a "war of necessity."

Following are questions and answers about the U.S. stakes and role in Afghanistan's second presidential election since the American-led military overthrow of the Taliban in 2001.

WHAT HAVE TOP U.S. OFFICIALS SAID ABOUT THE AUG. 20 VOTE?

Obama has withheld comment on U.S. ally and incumbent President Hamid Karzai and his nearest challenger, Abdullah Abdullah, to avoid charges of U.S. interference. Obama has shifted focus from Iraq to Afghanistan as his top foreign policy priority. Obama this week called the eight-year-old conflict "fundamental to the defense of our people" because of the risk that a successful Taliban insurgency could enable al Qaeda to operate more freely to plot another attack akin to the one launched against the United States on September 11, 2001.

Richard Holbrooke, the U.S. special envoy for Afghanistan and Pakistan, said the United States hoped the election would "reinvigorate, or invigorate if it's a different president, the leadership" of Afghanistan in tackling the country's many problems such as the opium trade, corruption and low economic development.

Defense Secretary Robert Gates acknowledged that Taliban threats to disrupt the vote created "adverse circumstances" for the election. But he said last week that there would be as many as 1,300 to 1,400 more polling places and several million more Afghans registered to vote than in the previous election in 2004, offering the potential for a "quite credible election" in all parts of the country.

FOR THE UNITED STATES, WHAT WOULD BE THE WORST OUTCOME?

The most troubling scenario would be an election outcome that is not seen as credible in the eyes of the Afghan people, diminishing their hope in the future and their support for U.S.-backed alternatives to Taliban rule, analysts say.

An election marred by excessive violence could produce a negative reaction by the U.S. Congress and erode American public support for the Afghan war, which has started to slip as U.S. troop deaths have risen this summer. After 44 U.S. troops were killed in Afghanistan in July, making it the deadliest month of the war for the U.S. military, a CNN/Opinion Research Corp poll showed U.S. popular support for the war at an historic low, with 54 percent opposed to the war and 41 percent in favor.

In a race in which Karzai is thought to be leading Abdullah by a narrowing margin, Brookings Institution foreign policy analyst Michael O'Hanlon said neither man represented "a terrible outcome" because each has strengths Washington could build upon in working with the next Afghan government.

Karzai relies on the backing of powerful -- and in the case of Uzbek militia leader Abdul Rashid Dostum, controversial -- warlords. Some U.S. analysts have said a victory by Karzai would be problematic if he rewarded warlords with Cabinet posts.

WHAT ARE THE NEXT CHALLENGES FOR THE OBAMA STRATEGY?

If no candidate wins more than 50 percent of votes cast, there would be a run-off election pitting the top two candidates, provisionally set for October 1. Analysts have expressed concern that a run-off would prolong the uncertainty in Kabul and require U.S., NATO and Afghan forces to maintain election security for more weeks in the face of Taliban threats.

Obama has deployed 30,000 extra U.S. troops, raising the level of American forces in Afghanistan to 62,000. Obama may face calls in the future from his generals for more troops, some analysts say. The request would come after General Stanley McChrystal, the new U.S. commander in Afghanistan, concludes a review of counterinsurgency strategy.

(Editing by Will Dunham)