The blast five kilometres north of the city of Hilla also wounded 20 people, a police captain and a doctor said.
Iraq is suffering a protracted surge in bloodshed that has killed more than 2,750 people this year, and the April 30 parliamentary vote -- the first since American troops departed in 2011 -- will be a major test for security forces.
They were able to keep violence to a minimum during provincial elections last year, but security forces have failed to halt a subsequent year-long surge in unrest.
It has also been fuelled by the bloody civil war in neighbouring Syria, which has bolstered militant groups.