Violence Looms 3 Days Before Afghan Vote

Campaigning must end at midnight Monday for the candidates in Afghanistan's presidential elections, just three days before Thursday's vote.

CBS News chief foreign affairs correspondent Lara Logan reports that an estimated 17 million Afghans are eligible to cast ballots, but how many of them will be able to get safely to the polls is unclear, especially in the Taliban strongholds of southern Afghanistan.

U.S. Marines in the hostile Helmand province continued battling Taliban militants over the weekend, firing mortars into a valley still under the insurgents' control.

Lara Logan's Reporter's Notebook on the tremendously difficult circumstances facing U.S. Marines on the ground in Helmand.

This area, the Nowzad valley, has been the site of intense fighting between militants and Marines as the U.S. tries to establish enough security for local people to come out of their homes and vote safely on Thursday.

The Taliban have vowed to attack polling sites and threatened to kill anyone who goes to vote.

Brigadier General Larry Nicholson is the most senior U.S. Marine in Afghanistan, and commander of all the troops in the south. He's adamant that votes will be cast in the restive region, but not overtly optimistic as to how many.

"Yes, there will be an election here. And it may be modest, but, you know, as far as I'm concerned, as long as one person comes out to vote... then that's a success," Nicholson told CBS News.

Security is tightening across the country as the campaigning enters its final moments.

Sunday night, the top three presidential contenders, including President Hamid Karzai, took part in an American-style televised debate.

Karzai leads in all of the latest polls, but not by enough. He'll likely face a runoff against his strongest challenger, former Foreign Minister Abdullah Abdullah.

President Karzai's security chiefs announced that the Taliban have agreed to allow more polling stations to be open in the south, but the Taliban quickly denied any deal had been struck.

In taliban controlled areas of the country, there are no ballot papers and no polling stations to open.