Well, he might just be positioning himself as the best gop pick for veep. The gop won't win just on moderates and independents, it needs the nutroots invested as well. They win when they appeal to emotional wedge issues, a sense of shared (if contrived) values, and common resentments.
Well yes....sort of. It's true that a candidate needs their base but the nation is pretty evenly split between Republicans and Democrats. It jockeys around a bit depending on what is going on but for the most part each party gets about a third or so of the electorate. Even with a highly charged base though, that's not enough to win an election. You MUST get support for the Independents. So the traditional method is to campaign hard toward your party base to secure the nomination, them immediately pull a u-turn and move back to center to get the Independents.
The theory is that once the nomination is secure going back to center is safe because the party base will suck it up and vote for their party's candidate whether they are totally in love with him or not because it's better than letting the other party win. With the extreme rhetoric Santorum is laying out lately on social issues I am not sure he is going to be able to effectively move back to center and gain Independent support.
After last night and what I think will be a big night for Romney next Tuesday I think it will end up being academic, but...well....there it is