In the latest attack directly linked to the polls, armed men on Sunday ambushed parliamentary candidate Adiq Ullah as he was driving in Uruzgan province, killing him and wounding two others in his vehicle, said provincial governor Jan Mohammed Khan.
He blamed the Taliban for the murder. Security forces pursued the insurgents, but they escaped, the governor said.
Ullah's killing brings to four the number of candidates killed in the lead-up to the polls. Four election workers have also been murdered and several election offices have been rocketed.
On Friday, insurgents attacked the US service members as they were patrolling about 40km east of Kabul, a US military statement said. An attack helicopter rushed to the site, but the attackers had fled.
The wounded were in stable condition after being evacuated to Bagram, the main US base in Afghanistan, about an hour's drive north of Kabul, the statement said.
Attacks on the US military so close to Kabul are rare, and Friday's assault occurred less than a week after a roadside bomb in the capital blew up near a convoy of US embassy vehicles, wounding two American staff members.
The assaults come amid a major upsurge in attacks by Taliban-led rebels that have left more than 1100 people dead in the past six months.
They also come less than a month before landmark legislative elections, which the Taliban has vowed to subvert, although it said it would not attack polling stations.