Among their first demands -- in alliance with religious parties from other communities -- has been to make Islamic law the sole source for Iraq's legal code.
That demand, opposed by secular forces including the Kurdish bloc in the National Assembly, today remains one of the major points of contention holding up the drafting of Iraq's first post-Saddam Hussein constitution.
But over the past weeks, the Shi'a religious parties have raised the stakes still further as Iraqis try to define what kind of country they want to live in.
Abd al-Aziz al-Hakim, the leader of one of the two largest Shi'a religious parties, the Supreme Council for the Islamic Revolution in Iraq (SCIRI), electrified a crowd of supporters on 10 August by calling for the creation of a Shi'a autonomous region.
"Regarding the central-southern region, we think that it is necessary to form one entire region for central and southern Iraq, due to the common characteristics of the residents of these parts and the same unjust policies which were adopted against them," al-Hakim said.
The crowd in the Shi'a holy city of Al-Najaf responded with shouts of praise, because the call promised an autonomous region in which the religious parties could well emerge as the ruling power. If so, supporters of Islamic law could see it gain dominance regionally, even if it founders nationally.
Other Iraqi politicians have criticized the proposal as threatening Iraq's unity. The proposal would group nine Shi'a- majority provinces -- half of Iraq's total provinces -- into a single self-rule area.
The reaction has been particularly severe from Sunni parties. They fear autonomous Kurdish and Shi'a regions could hoard Iraq's oil wealth, leaving the Sunni center of the country -- which has no oil fields -- impoverished.
Shi'a secular parties and even the other large Shi'a religious party, Islamic Al-Da'wa, have also called the idea "unacceptable."
Prime Minister Ibrahim al-Ja'fari, who is from Islamic al-Da'wa, distanced himself from the idea. His spokesman, Laith Kubba, gave this official response immediately after al-Hakim's call: "I do not think that the principle of government structures built on ethnic or religious grounds is something we need. Indeed, there may be specific features of some governorates who may unite and establish larger administrative units if they want [to boost their] economic revival. But I do not think that the idea of a Sunni, a Shi'ite, or a Kurdish region has been generally accepted."
But such criticism has done nothing to discourage SCIRI representatives from formally pressing their autonomy proposal in the constitutional negotiations. There it threatens to eclipse the debate over the extent of Kurdish self-rule as the principle issue in defining a new federal Iraq.
Now, over the past few days, al-Hakim has added yet another demand: a special status within Iraq for Al-Najaf and the other major Shi'a holy city of Karbala. That would duplicate the independent status the Vatican -- the center of the Roman Catholic Church -- has within Italy.
As SCIRI inserts its multiple demands into the constitutional talks, some analysts say the party is deliberately amassing bargaining chips for the last-minute negotiations ahead of the 15 August deadline for completing the draft charter.
"They are talking about [an autonomous region comprising] half of Iraq," said Alireza Nourizadeh, director of the London-based Center for Iranian-Arab Studies. "And I think the other parties -- Kurdish, Sunni, the secular forces -- they would not accept this idea. And I think Mr. Hakim may at the end of the day use it as a bargaining chip to get other things that he wants the constitution to contain."
But what does al-Hakim really want?
Nourizadeh said the priority might be to get Baghdad to delegate as many powers as possible to the Shi'a majority areas, even if that falls short of formal autonomy. That, in turn, would increase the religious parties' potential for controlling the region if they can gain political dominance.
The Shi'a religious parties already have amassed considerable power in Al-Basrah and the surrounding province where they have governed for more than a year. The parties have sought to impose Islamic values, including closing alcohol shops and encouraging women to wear head scarves.
Nourizadeh suggested that part of al-Hakim's strategy might also be to secure a strong future financial base for his party by gaining control over revenue sources in Al-Najaf and Karbala.
"Now Mr. Hakim would like to have it guaranteed that there would be a special status for Najaf and Karbala, the two most holy cities for Shi'a, and [have] the two towns run by the Marjariya [the Shi'a religious establishment]," Nourizadeh said. "It means that the city would fall into the hands of SCIRI and the Al-Da'wa party and they will control these two cities which are very important cities. Thousands and thousands of pilgrims visit these cities and the amount of wealth which is going to come to these cities will fall into the hands of SCIRI and Al-Da'wa."
But it remains an open question whether such goals are achievable. Much of the willingness of other members of the constitutional committee to consider SCIRI's demands will depend on their perception of how strongly supported they are by the Shi'a population.
Many observers say there is widespread sentiment among Shi'a that they have suffered under centralized rule from Baghdad, particularly during the Saddam Hussein area.
There is also feeling that the continued distribution, for example, of much of Basra's budget through Baghdad, keeps the south at an economic disadvantage today.
But whether those complaints translate into popular demands merely for more local powers or for full autonomy is unknown, because no widely known polls have been taken.
That means the bargaining on the constitutional committee is sure to be hard over the coming days, with little data to back the conflicting claims.