As the old saying goes, "the trend is your friend."
Rick Santorum has been a second tier candidate since the race began. What happened to him in Iowa was the result of massive negative campaigning against Newt by the Romney camp. It's a spike that will not last, and is even now, settling back to normal.
This is also why you're beginning to see Newt's numbers rebounding. Gotta go, but keep those things in mind.
On one of the news shows last night I saw one poll with Gingrich in 2nd and one poll with Santorum in 2nd in SC.