Also Monday, a suicide car bomber blasted a police checkpoint in the western city of Ramadi, killing seven policemen, an official said.
The two incidents in widely separated parts of the country illustrate the dangers still facing Iraq despite a sharp decline in violence over the past year.
The attack on the bus occurred when gunmen opened fire as their car passed it in the Abu al-Khasib area south of Basra, police and election officials said. A third election employee was wounded.
The dead included the head of a local government committee preparing for provincial elections, Maath Wahab, and his deputy, Jassim Mohammed, according to Hazim al-Rubaie, director of Basra electoral committee.
No group claimed responsibility and no arrests have been made. But local officials said Wahab was known for resisting interference in the electoral process by Shiite religious extremists.
The officials spoke on condition of anonymity because they feared for their own safety.
Provincial elections are expected late this year and will likely redistribute power among Iraq's political and ethnic groups. No date has been set because legislation has been stalled in the national parliament, but preparations have begun nationwide.
Control of polling places would enable parties to manipulate the results.
Voters will select members of the provincial councils in Iraq's 18 provinces. Under the 2005 constitution, those councils will wield considerable power over security and resources, including Basra's vast oil wealth.
Basra had been under the control of rival Shiite militias until Prime Minister Nouri al-Maliki launched a military operations last March that wrested control from the gunmen.
The suicide bombing happened about 9:15 p.m. in the Tamim area about three miles west of downtown Ramadi, capital of Anbar province 70 miles west of Baghdad.
Ten people - six policemen and four civilians - were wounded in the blast, according to police Maj. Gen. Tariq Youssef.
Ramadi was once the deadliest city in Iraq for U.S. forces when al Qaeda in Iraq and its allies held sway over the city. The militants' grip ended when Sunni Arab tribes revolted against al Qaeda last year, but attacks still occur.
U.S. forces had planned to hand over security responsibility to the Iraqis last month but the transfer was postponed indefinitely after a suicide attacker killed three U.S. Marines and 20 Iraqis in the Anbar town of Karmah.
Elsewhere Monday, a roadside bomb exploded in Mosul as a convoy carrying the city's mayor sped past, police said. The mayor escaped injury but a bodyguard was wounded, police said.
U.S.-backed Iraqi troops have been seeking for months to rid Mosul of al Qaeda and other Sunni militant groups.
In Baghdad, mourners in the city's Azamiyah district fired weapons in the air in a display of grief during a funeral for Farooq al-Obeidi, deputy head of a group of U.S.-allied Sunni fighters who was killed by a suicide bomber.
Nine other people were killed and at least 20 were wounded in Sunday night's blast, Iraqi officials said. They declined to be identified because they weren't authorized to release the information.
The bombing was significant because it occurred in the heart of the northern Baghdad neighborhood of Azamiyah, which has been surrounded by a concrete wall built by the U.S. military to stop violence. That makes it difficult to smuggle explosives in from other parts of the city.