A car bomb followed by a suicide attack hit the campaign rally for the Sadiqun bloc, the political wing of the Asaib Ahel al-Haq militia, interior ministry spokesman Brigadier General Saad Maan said.
Dozens more were wounded, he added.
The blasts came at around 5:30 pm (1430 GMT) near the Al-Qanat highway through eastern and northern Baghdad. Apparently Qais al-Khazali, leader of Asaib Ahel al-Haq, or League of the Righteous, was present.
The rally came with campaigning at a fever pitch ahead of Wednesday's polls, Iraq's first since March 2010 and its first national elections since US forces withdrew in late 2011.
A number of Shiite blocs are battling Maliki for votes in his traditional heartland of central and southern Iraq.
They include Sadiqun but also the Ahrar movement, which is linked to powerful cleric Moqtada al-Sadr, and the Citizens bloc, a formerly powerful political group seen as close to Iran.