The future of this is now pretty clear: It’s going to work just as the original Medicaid roll-out did. That was also optional for states, and many of them declined the first time around, but eventually all 50, no matter how conservative, found themselves participating. The key — and I expect this to be true of the ACA Medicaid expansion as well — is that the decisions were one-way. Over time, some of the decliners decided to join, but no state walked away.
The remaining question is: Will Republican governors pay any price in national politics for accepting Medicaid expansion? For any governor who has national ambitions, the hope has to be that the expansion rapidly shifts from a betrayal of Republican principles to something that almost all the states are doing. Otherwise, it’s almost certainly going to be a weapon used against them.