Fortunately for proponents of CIR, these mutually incompatible calculations have led both parties to support the same policy. But nothing about the policy itself has altered the physics of two-party politics. Someone's math is wrong.
My hunch is that Republicans are wrong. That they think appealing to growing Democratic constituencies with policies the business community already supports will be an easier way to remain electorally viable than rethinking their broader economic agenda, and thus have a blinkered view of what CIR will mean for American politics in the future.
But maybe I'm wrong. Maybe Democrats are just being hubristic. It's hard to say with certainty, because it's a genuinely complicated calculation. Ezra alludes to this in his post, but gaming out the political consequences of immigration reform isn't straightforward. The politics look different in the near term than in the long term. They'll impact national politicians in different ways than they'll impact candidates for state-wide or local offices.
But over a long enough time horizon, there will be one winner. A majority of new citizens will either be Democrats or Republicans. To the extent that the new GOP position on immigration reform changes existing voters' minds about politics, only one of two parties will be on the winning side of that realignment.
Some important Republican strategists and opinion makers recognize this, and worry the GOP has picked a loser. And one of the things that's helped CIR maintain its pulse on Capitol Hill is that these voices haven't persuaded party leaders. At least not yet.