There biggest potential sticking point to passing a broad immigration reform law could be cost and the biggest possible driver of cost could be how immigration reform would interact with the Affordable Care Act.

Currently a bipartisan group of senators seems to have reached an agreement on broad outline but it is easy to support shared principles until money is involved. Once details immigration reform start getting worked out it will inevitable becoming entwined with the two biggest recent fights in Washington, Obamacare and the deficit. This will greatly compound the difficult of getting a final deal

As it stands the roughly 11 million undocumented immigrations are completely left out of the ACA. They were purposely not given access to the Medicaid expansion or the exchange subsidies so they won’t cost the federal government any money under the new law to keep the price tag low. If these immigrations having their status legalized it inevitable creates the question of how, if, and when these individuals should get access to the ACA.

I’ve not seen an official analysis but my rough estimate is that giving this population full access to ACA could increase the cost of the law by several billion a year. That is a significant amount of money especially for a Congress irrationally obsessed with deficit reduction at the moment.

The bipartisan proposal seems to skirt this issue in the short term by continuing to deny “probationary immigrants” access to federal benefits. Long term though this is not only inhumane it is also practically untenable if there is going to truly a pathway to citizenship. By design eventually many will become citizens and have access to federal benefits.

Congress could try to punt on this ACA issue by gaming the CBO score. The only way to do that is to effectively denying this group full legal status until after the CBO’s 10 year window, but consigning millions to over a decade of legal limbo just to save a few dollars could be too much for some Democratic politicians and activists to swallow.

At some point in the negotiations this issue will need to be addressed and it unfortunately has the potential to force many people from the table. Either as a result of honest unbridgeable differences or as a convenient excuse for failure.