This is a little of what he told Mark Levin (as I wrote it up earlier in the month):

Although McCain has a largely pro-life voting record, Santorum — who led on pro-life and marriage issues in the Senate — cautioned against misunderstanding McCain’s public stance: “Not only was he wrong on embryonic-stem-cell research, but on a whole host of conservative issues, where he may have voted with us.” Santorum took radio listeners into the back-room workings of the Senate, emphasizing how the first step toward legislative success is finding time on the floor to discuss and vote on the issues: “That discussion is held in private, where you’re jostling and jockeying to get your legislation into the queue so that you can have your time on the floor to get something done. And I can tell you, when social-conservative issues were ever raised — whether it was marriage or abortion or a whole host of other issues — there were always the moderates who said ‘no, no, no, we can’t: they’re divisive, divisive, divisive.’ And more often than not, John McCain was . . . with them,” agreeing that these were divisive issues that the Senate should not bring to a vote.
“That’s wrong,” Santorum added, “and that gives me an insight into what he would really be like [on these issues] if he were president of the United States.”