Richard Nixon is the best parallel I can think of from the last century. Nixon adopted or proposed a host of liberal initiatives -- affirmative action, wage/price controls, even a guaranteed annual income. He did so, I believe, out of indifference. Nixon's goal with respect to domestic policy was to remain sufficiently viable politically to conduct American foreign policy.

There's some of this in McCain. Foreign and national security affairs are his passion, and he cares little about social issues. However, he is hardly indifferent about matters such as government spending, immigration, and clean government. Motivation and interest aside, a McCain presidency would likely resemble Nixon's in that he would combine hard-line foreign policy with some centrist or liberal domestic policies. But because McCain is far more principled, he surely would be more resistant to a broad liberal agenda than Nixon was.