BBC NEWS | Middle East | Iraqi parties form election blocs
The main Sunni parties that boycotted the vote in January have set up a coalition, the Iraqi Accord Front.
The US hopes Sunni participation will sap support from the insurgency.
Iraq's political process received a boost this week with the announcement that voters had backed the new constitution in the 15 October referendum.
The result came despite strong opposition from the minority Sunni community.
The new Sunni alliance, announced on Tuesday, called on Iraqis to take part in the December's poll and to reject any calls for a boycott.
They want to increase Sunni representation in the national assembly, which is dominated by Shia and Kurd parties partly because of the Sunni boycott in January.
The ruling Shia Islamist parties only agreed to register as a united bloc after a last-minute agreement on Thursday evening.
"The United Iraqi Alliance will be maintained," said Jawad Maliki, of Prime Minister Ibrahim Jaafari's Dawa party.
The three principal Shia movements - the Supreme Council for the Islamic Revolution in Iraq (SCIRI), the Dawa party, and the movement led by Shia cleric Moqtada Sadr - settled a deal despite reports of power struggles and rivalries.
Militias linked to SCIRI and Mr Sadr have clashed with each other in southern Iraq in recent months.
The two main Kurdish parties - the Democratic Kurdistan Party and the Patriotic Union of Kurdistan - will also be running together, they announced.
A fourth challenge for votes will come from the Shia former Pime Minister Iyad Allawi, who has expanded his secular list to include Sunni personalities, as well as communists and liberals.
This centrist bloc hopes to capitalise on the perceived failures of the current transitional government, and to provide an alternative for voters opposed to sectarian politics and violence, the BBC's Jim Muir in Baghdad says.