Mark Thiessen, WaPo:

Instead of putting Gingrich on the defensive for channeling Michael Moore, Romney spent not one but two debates on the defensive over releasing his tax returns. At the conclusion of Thursday's CNN debate, Romney was asked if he had any regrets about his campaign. He replied that he regretted the time he had spent talking about his opponents instead of focusing on Barack Obama. Wrong answer. His mistake was precisely the opposite. When Romney had the opportunity to attack Gingrich from the right and deliver a devastating coup de grace, he demurred. It was his "Pawlenty moment."

With a free pass from Romney, Gingrich shined in the South Carolina debates and used them to right his faltering campaign. He used moderators Juan Williams and John King as foils, declaring to cheers from the audience: "I am tired of the elite media protecting Barack Obama by attacking Republicans." He deftly put the Bain debacle behind him and rallied conservatives behind his candidacy once again.
And Romney stood there helplessly and let it happen.

Gingrich was nimble and flexible in the face of adversity. Romney was stiff and flatfooted — and he lost the South Carolina primary as a result.

The damage to Romney's campaign could extend beyond South Carolina for this reason: The central premise of Romney's candidacy is that he is the best man to beat Obama. But in South Carolina, Gingrich borrowed directly from Obama's playbook, launching the exact same attack Obama will use against Romney this fall if he is the nominee. Romney responded with all the agility of a deer caught in headlights. He had a chance to show just how he would take the fight to Obama in November — and he failed miserably.

This should raise a question in the minds of GOP voters: If Romney can't defend free-market capitalism against Gingrich, how will he be able to defend it in the fall against Obama?

What I don't understand — well, aside from his icky social conservatism, which is an existential threat to the nature of the contract between the individual and the government in a way far more worrisome than, say, advocating for government/private partnerships, or waxing rhapsodic over the moral and ethical necessity of ethanol subsidies, or identifying politically with Woodrow Wilson and FDR — is why the conservative base didn't swerve to Santorum, who didn't attack Romney on Bain capital, who has never loved the idea of an individual mandate, who doesn't believe in the "grandiosity" of the government or government leaders, and who has performed quite well in the debates on both substance and in defending conservative (if not always libertarian) ideas.

While Gingrich tosses the red meat and support for him seems to swing wildly depending upon just that ability, Santorum has remained steady throughout the campaign.

Don't get me wrong: I'm glad that Gingrich has weakened the "inevitability" of Romney (whose campaign pundits disagree on how best to salvage) and I would happily vote for him over Obama, with the hope that Gingrich's ego and sense of history would cause him to adhere strongly to the conservative message, now that he senses that the ethos demands such a tack. As I've said of Gingrich from the beginning, he could be a very successful conservative or a very successful progressive, depending upon which way the political winds are blowing.

But frankly, I'd be much happier with a candidate who sees the Presidency as a position that serves the people, and who approaches government not as the realm of philosopher kings filled with big ideas, but rather as a collection of representatives working within the carefully established constraints of the Constitution to best serve and protect the governed.