In the late stages of the South Carolina primary, the campaign is still so intense that things could shift. Mitt Romney was an unimpressive front-runner in last night’s debate, while Newt Gingrich made his supporters feel good about their decision to be for him. All the other candidates are sinking. So, unless Romney has an aggressive close planned for his South Carolina effort, he is vulnerable to making a mistake that could make Gingrich his only credible challenger and the beneficiary of the end of the Santorum and Perry campaigns. Ron Paul will also be a factor, but he has seen his peak, and his support is no longer growing in South Carolina.

If Gingrich is on his game — and last night’s debate performance suggests that the new Newt is back — today will mostly be about Romney’s tax returns. Bain is over as an issue. The return of the “good Gingrich” coincides with the appearance of an issue that can be used against Romney that may be effective. Suddenly, Romney is facing a bumper sticker that says “Release Them,” and he does not appear to be ready to publicly deal with the contents of his tax returns. His lead may not be strong enough to withstand four days of Gingrich calling for openness and transparency and making a compelling case that President Obama will pounce on Romney and build unbearable pressure for when the release date comes. And it will come.

South Carolina isn’t locked down for Romney if Gingrich has a good day and Romney gets flustered or gives Gingrich the gift of even a small gaffe. Romney must not take his lead for granted; he can’t make a mistake because he has an able Gingrich stalking him and he has some explaining to do. It’s not a friendly scenario for the last few days of a campaign. It’s not over until it is over.

And, oh by the way, keep an eye on gasoline prices. This Insider will continue to talk about the political significance of gas prices. The issue could be poison for Obama.