Who do you think will win?

I don't think the endorsement is likely to help Romney with anyone who wouldn't vote for Romney anyway, and it might actually have a slight negative impact with some independents on the fence about Romney but turned off by an extremist like Santorum. It does demonstrate Santorum's willingness to be a team player, which could improve the party's attitude toward him when he runs again in 2016. But I'm honestly not sure whether Santorum is actually trying to help Romney by endorsing him, which would require both that he think his endorsement would help and that he actually want Romney to win. (A Romney win would not be good for Santorum's prospects in four, or even eight, years.)