Hope for passage.
The alternative is another interim government.If they are to have civil war, they will have it anyway.
If this passes, even if the government itself proves unworkable, the Iraqis will have one example to look at to prove to themselves that they can have an election that works, and nobody need be killed. If they can do it once, they can do it again.
That kind of example gives hope, and hope is what cooperation is built on.