Romney is in a good place - he has the momentum. All he needs to do on April 24th is keep Santorum from gaining any momentum and keep it for himself.
How does he do that?
Downplay expectations in Pennsylvania. Spend about a third of your time in Pennsylvania, the rest on making sure you carry all the other states voting that day, which you're expected to win. If all the attention is on Pennsylvania, if Santorum wins, he'll grab the headlines and thus, the momentum. You can win the headline war if you win everywhere else and focus more attention there. A moderate Santorum victory with Romney not campaigning hard there would be much better for Romney than a razor-thin margin with Romney trying hard.
Momentum doesn't matter at this point. All that matters to Romney is a) not making a colossal gaffe and b) getting his remaining opponents out of the race so he can focus on November. It doesn't really matter much if Santorum wins PA and it's unexpected, but it would be nice if he lost and dropped out.
