ABC News

The assumption was that Saturday's ABC/WMUR debate was going to produce some serious fireworks.

After all, you have a front-runner who is quickly assuming the mantle of the "inevitable" nominee. If ever there was a time for his rivals to try and change the trajectory of this race, this was it.

Instead, Newt Gingrich, Rick Santorum and Jon Huntsman took only glancing blows at Romney, while Ron Paul battled with Santorum and Gingrich.

So, why was the conventional wisdom so wrong?

1) It's a lot easier to attack someone when you aren't standing three feet away from him. Rick Santorum was exactly where he said he's always wanted to be in this campaign. In the center of the stage and in the spotlight. But, he also discovered, as many others have before him, that with that new position on stage comes additional and unexpected pressures. Making an attack on the campaign trail is easy. Saying it to someone's face is hard.

2) The Likability Factor:  For months now, the focus has been on which candidate can ultimately become the "not-Mitt" alternative. And, every week it seems, there has been a different alternative. But, voters want to do more than vote against a candidate, they want to vote FOR someone. Attacking Romney would only remind voters about what they disliked about a candidate instead of what they liked about him. Moreover, when Gingrich and Santorum go on the attack, they can come across as snide or mean, the opposite of likable.

3) The Future:  Specifically, candidates on stage had to be thinking of their political future. Did Gingrich want to be known as the candidate that did a slash and burn job on the eventual nominee or that his biting attack on Romney was turned into a devastatingly effective ad for Obama?

Don't be fooled, just because the candidates held their fire tonight doesn't mean that they won't be battling Romney going forward. In fact, some already have their surrogates - the SuperPACs - doing the attacking for them in South Carolina.