Mr. Santorum likes to argue that “Governor Romney doesn’t provide the contrast we need to beat Barack Obama” on issues like health care and cap and trade, and he’s on to something. But he’s yet to take his own point and focus on the sweeping economic contrast voters are aching to hear. To the extent Mr. Santorum makes that argument, it often comes second to his cultural pitch.

Even when he does, the contrast isn’t as clear as it might be. A manufacturing tax credit isn’t a campaign theme; it’s a bullet point (and one, for the record, Mr. Obama largely agrees with). The opportunity for contrast is in a sharp economic message that directly challenges Democrats on class warfare, taxes and size of government. Such a message would also help Mr. Santorum to outflank Mr. Gingrich, who remains the biggest threat to a conservative Santorum coalition.

And it would help with his third big challenge—defense. Team Romney is now turning its big guns on the former senator, who has benefited by staying above the Romney-Gingrich fray. The Santorum folks are betting Mr. Romney is getting a reputation for slash-and-burn, and that this will hurt him. He is, and it will, but the attacks won’t stop. And criticism that is left unanswered has a way of seeping into voters’ minds.