Santorum's immediate challenge is: to push Newt Gingrich completely out and to consolidate the hard right behind him.
"If Santorum and Romney are facing each other head to head, it's very likely that they are going to divide up the remaining delegates in a way that would lead neither one of them with enough strength to claim the nomination on the first ballot," Professor Matt Kerbel of Villanova University said.
Despite his successes, Santorum's path to the Republican nomination remains very narrow and still, the experts say, beyond Santorum's reach.
"There simply aren't enough delegates left to be chosen and Romney has enough of a lead at this point to make it very difficult for Santorum to overtake him." Kerbel said.
Pennsylvania voters drove Santorum out of the U.S. Senate 6 years ago by an 18 point margin.
What do Republicans think of his rise to national prominence?
"I think he's good on foreign policy, good on economics, and I think he's good on social issues," Scott Steward of Fort Washington said.
"I think he's a little bit too extreme for the party today for my liking," Dave Sternberg of Elkins Park, Pa. said.
The Santorum-Romney smackdown will play out into the summer, long past Pennsylvania's late April primary.
The polls show the former senator with a huge lead, one expected to serve him well in his continuing underdog race against Romney, the struggling front-runner for the GOP nomination.