The comments came from Abd al-Aziz Hakim, whose Shia-dominated coalition of parties is expected to cruise to victory in the polls.
His comments, taken as the most outspoken by a Shia leader to date, concerned Iraq's Sunni Muslims who are expected to stay away from the polls due to political disenchantment and fear of violence.
"Whether the Sunnis have many or few seats in the next parliament, we need the real participation of all," he said.
"All must participate in the governmental authority and we will
insist that they be represented in the government, that they have posts and they should also have a voice in drafting the constitution and responsibilities in the government. This is what we are seeking to achieve."
Earlier in the week, Hakim, the head of the Supreme Council of the Islamic Revolution in Iraq (SCIRI), and his coalition of Shia parties urged Sunnis to vote, but stopped short of any explicit promise of seats in the next government.
Hakim tops the Unified Iraqi Alliance, a 228-strong candidate list of Shia parties. He is poised to lead the country's 15-million-strong religious majority which is widely expected to solidify its grip on power in the vote.
US and Iraqi officials are worried that if Sunnis fail to turn out at the polls it will drive a wedge between the two communities and further strengthen the fighters.