Santorum went from an also-ran to a front runner.
Not really. He doesn't have deep support outside of evangelicals and the party establishment doesn't want him. This is his fifteen minutes.
Yet when the Iowa cacaus rolls around Santorum vaults past both Gingrich and Paul and even past Romney for most of the vote count. For Santorum losing Iowa by just 8 votes is a massive victory. That clearly makes him a frontrunner.
He doesn't have the funding, appeal, or staying power to be considered a frontrunner. If he places second in SC or even wins there in an upset, it'll be different. Everyone in this thing has had their mirage--a surge of support that makes them a hot story, then fades.