But the damage was done, and the Gallup national survey put Romney back in the lead. The Associated Press currently estimates that Mitt Romney has won 53% of the Republican delegates awarded so far. To block Romney on the first ballot in Tampa, his opponents need to hold him below 40% of the delegates in the remaining contests. That will be difficult: Romney is likely to score big in California, New York, New Jersey, and Utah. It’s not impossible. Complicating this task are Newt Gingrich and Ron Paul — still splitting the vote three ways, but fading.

Santorum’s defeat of Gingrich in the Southern primaries gives him one last chance to rise. The CNN exit polls in almost every state so far have shown that conservatives outnumber moderates by an average of roughly 60% to 40%. Even in Massachusetts, the only state to support George McGovern 40 years ago, conservatives represented 51% of Republican voters.

The key to the next few weeks: can Santorum win over fiscal conservatives in the suburbs of Chicago on March 20, and in Milwaukee on April 3? If Romney wins another close election in a big Midwestern state, he will likely coast to a majority of delegates by June.

Santorum still has a slight chance of winning the GOP nomination in 2012, but his chances were greatly diminished by his mid-winter mistakes.