Matt Lewis of the Daily Caller writes:

Barack Obama and his team have spent months assuming Mitt Romney will be their opponent. But while Romney is still the favorite to win the GOP nomination, the surging Rick Santorum might be a much tougher opponent to defeat.

This is true for a variety of important reasons — many of which overlap:

...4. Values voters. In the post-Bush era, there has been a backlash against so-called “compassionate conservatism.” While conservatives were correct to repudiate the big-spending days of the 2000′s, traditional conservatives haven’t suddenly become selfish Ayn Randians, either. James Pethokoukis picked up on this recently, comparing Santorum’s Burkean philosophy to Mitt Romney’s more Hayekian economic stance. He writes that Santorum

isn’t satisfied with an economy that’s more efficient and competitive if it doesn’t result in stronger families. As it says on his campaign website: “Rick Santorum believes that to have a strong national economy, we must have strong families.” The family is at the center of Santorum’s economic vision. GDP growth is a means, not an end.

Santorum’s focus on cultural issues — in an era when they have been downplayed by many Republicans — has made him a very appealing candidate to Evangelicals.