One week ago, Newt Gingrich was on the ropes in South Carolina, under near-universal assault on the right from his attacks on Mitt Romney’s record at Bain Capital. Everyone from Rush Limbaugh to the Club for Growth and the Wall Street Journal had all declared their disgust. The conservative backlash had given Romney a double-digit lead in the polls. At a candidate forum hosted by Mike Huckabee, Gingrich was booed by the crowd when he tried to defend his Bain attacks.

Fast forward one week, and Gingrich is the winner of the South Carolina primary. Not only did Gingrich win, he crushed Romney 40.4 percent to 27.9 percent — a swing of more than 20 points in just a week. And a new poll shows that, after trailing Romney by more than 20 points in Florida last week, Gingrich has now opened an eight-point lead in the Sunshine State.

What happened? Simply put, Romney let Gingrich up off the mat.

In his concession speech after Saturday’s loss, Romney laid into Gingrich, accusing him of “demonizing success” and using the “weapons of the left.” Romney declared, “If Republican leaders want to join this president in demonizing success and disparaging conservative values then they’re not going to be fit to be our nominee,” adding “We cannot defeat the president with a candidate who has joined in that very assault on free enterprise.”

It was the right message, delivered a week too late. If Romney had made those very points in the South Carolina debates he might have been the one who received the standing ovations instead of Gingrich — and might well have emerged the winner in the Palmetto State.

But instead of seizing on the conservative anger over Gingrich’s attacks and going in for the kill, Romney gave Gingrich a pass. 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?

Romney’s debate strategy of staying above the fray and focusing on Obama while the lesser creatures of the forest fight among themselves has run its course. This approach may have worked when there were eight or nine candidates on the stage. But now there are just four — and only one, Gingrich, poses an existential threat to Romney’s presidential aspirations. Romney and Gingrich will meet in at least six more debates before the primaries are over — starting Monday night at Florida State University. If those debates go the way the South Carolina debates did, Romney is in deep trouble.

Romney may have outlasted Tim Pawlenty, Michele Bachmann, Jon Huntsman and Rick Perry. But he will have to defeat Newt Gingrich to win the GOP nomination.