Senator John McCain opposed the Federal Marriage Amendment during President Bush's first term on the grounds that such decisions should be left to states (meaning state legislatures or voter initiatives, not state judiciaries). To be consistent with that position, he will probably now support a similar but not identical amendment to prevent state judiciaries or the federal judiciary from interfering with the process that he originally favored. So this position that I'm predicting will be entirely consistent with his original one, even though in one instance he opposed an amendment to the Constitution and in the other he would be favoring one. How much do you want to bet that this is going to be played very heavily as a flip-flop?
Update: I thought it might also be worth a reminder that people have previously taken McCain's consistent view on gay marriage to be a flip-flop simply because he decided to distinguish between two different things people might mean by 'gay marriage'.