Opponents of the Iraqi draft constitution - a group which seems to include the majority of the Sunni negotiators and a splinter group of Shias - seem to have raised two controversial issues. One is the role of Islam in the constitution and the other is federalism.
It doesn't matter that on the role of Islam the constitution's critics are divided between secular Sunni leaders who claim that describing Islam as "a source of law" (significantly not THE source) is too great a role and hardline Shias who think Islam should be accorded a greater role. Both can still unite in voting no.
The more puzzling argument is over federalism. Oddly, it is the minority Sunnis who seem to favour a strong federal centre, while the majority Shias, along with the Kurds, support decentralisation. This is odd, because, unlike the dictatorships of the past, a democratic constitution will not allow the Sunnis to dominate the centre. Indeed, as long as Sunni intransigence creates an alliance between the Kurds and the Shias, Sunnis are unlikely to run anything except their own provinces in the centre. Even Baghdad has a Shia majority.
The Sunni fear seems to be and decentralised constitution will lead to a break up of the state of Iraq, leaving them with a barren, landlocked and oil-free desert. The Kurds would have the oil-rich north and the Shias the southern oil fields and ports. While such a scenario is possible, it is most likely to be triggered by a constitution that is too centralised, not one that is too decentralised. No-one thinks that the Shias - with 60% of the population - have any interest in breaking up Iraq. Likewise the Sunnis, unless oppressed by a strong centre run by the Shia majority, are unlikely to secede. The Kurds are much the most likely to want to break away. Any rolling back of the autonomy Kurds have had over the past 10 years would probably trigger secession. This, in turn, could very well lead to a war, and perhaps a split between the Sunnis and Shias as well.
The Sunnis would be far better off supporting a dectralised federal structure, and are being let down by misguided leaders who have not yet come to terms with the fact that the old Iraq, in which the Sunni minority bossed everyone else around, has gone for ever.