It's about time. Perhaps the campaign can now play out primarily between two serious, principled, decent men who are not using a career in politics to work out their own issues of ego and entitlement.
Mitt Romney is going to have to up his game. However he responds to the new Santorum threat, it would be a disastrous mistake to attack him from the left. Trust me, there will be plenty of liberals itching to do just that -- using video clips like this to try to paint Santorum as out of the mainstream. Better to hammer away at electability and "Washington insider" issues.
For Rick Santorum's part, his attacks on Romney from the right are cogent and seem to be working. With that said, he needs to find a way to capitalize on his victories last night. As a fellow social conservative (although not in 100% agreement with Santorum on all topics), I'd warn him that adversaries on the left will try to characterize him as a hate-filled monster. It's up to him to work hard to "humanize" himself; talking about his children (where his love for them is obvious and softens him wonderfully) and deploying his wife as a surrogate and character witness are effective ways to do this.
Finally, Republican voters need to realize that because of Santorum's courageous defenses of religious and/or conservative principles in a variety of hot-button topics, nominating him as their candidate in 2012 means that a lot of the debate this election year will center around social issues rather than economic ones. If that's what everyone wants to do, I'm in -- there are good arguments to be made. But everyone needs to be aware that it may be a riskier, rockier road.