Many religious conservatives want the marriage amendment passed. But McCain voted against it in 2004, when the Senate last voted on the measure. “[McCain] would be happy to appoint the likes of Chief Justice John Roberts to the Supreme Court. But he indicated he might draw the line on a Samuel Alito, because ‘he wore his conservatism on his sleeve.’”
It gets worse from there. He was against Minnesota Right-to Life to protect his McCain-Feingold bill. Amendments don't get passed easily, but legislation he is responsible for and supports can get passed easily. Legislation informs the SC.