Let us speculate on the fate of two cases, King v. Burwell (Affordable Care Act subsidies to health care exchanges run by the federal government) v Mata v Holder (same-sex marriage).
Given Justice Thomas’ dissent in the Alabama same-sex marriage stay some are speculating that Justice Roberts will vote to make it 6-3 in holding that the Constitution protects a right to same-sex marriage. Assuming that is correct, there may be a several possible reasons for his vote.
Obviously one answer is that he thinks this is the correct constitutional answer. Second, the Chief Justice is engaged in some smart politics and strategic decision making. Should he vote to rule that same-sex marriage is protected under the Constitution he is in the position to write or assign the majority opinion. He of course could assign the opinion to Kennedy (assuming which I do that he is the fifth vote) or he retains the right to pen the opinion himself, thereby giving him more leverage over what the Court opinion is or simply to ensure that the decision is at least 6-3, thereby strengthening its legitimacy.
Another scenario: He is leaning toward striking down the federal subsidies under the ACA. This is not an impossible scenario and given rumors that he switched his vote in 2012 regarding the constitutionality of the mandate, it is possible that now he write a 5-4 opinion striking down the subsidies on statutory interpretation grounds. Thus, voting to support same-sex marriage under the Constitution gives him some political capital or deflects away from his decision in Mata invalidating the federal subsidies.