You're starting to hear this a lot -- most recently, from Bill Kristol about 10 minutes ago.
"He [Romney] has not been subjected to a lot of negative attacks from his competitors on the Republican side... they've been busy cutting each other up.
.... We just don't know, honestly, how well Romney will stand up to attacks."
Last night, Karl Rove said the same thing.
"So far this season, people have been relatively ineffective in making an attack on Romney.
On the one hand, that's been an advantage to Romney, because they tried to lay a glove on him, and they haven't.
On the other hand, he hasn't had to confront these issues in a way that he's going to have to confront them in a general election."
We've only had a few glimpses of what Romney's like under attack this cycle, and it hasn't been particularly pretty.
Bret Baier caught him during his campaign's brief panic over Newt, and Romney bristled severely under Baier's questioning.
You get the sense that both Rove and Kristol would like to see a little more crisis management from Team Mitt before sending him to the major leagues.
The problem is that you can't exactly call Rick Santorum a crisis for Romney.