If Republican presidential debates mattered as much as everyone has said they do, the final days before Saturday’s South Carolina primary might now be seen as a political duel between Mitt Romney and Newt Gingrich. After Monday’s contentious forum here, Romney must hope that such forums have been overrated.

The 16th debate of the GOP primary campaign produced one of the strongest performances in weeks by the struggling Gingrich. In contrast, through much of the night, Romney, the clear front-runner and, to many Republicans, the inevitable nominee, seemed either defensive or evasive. He redeemed himself in the final minutes, but he must be happy that factors beyond the exchanges will determine the outcome of the race.

When Gingrich was down and almost out last fall, he used the debates to revive his candidacy. Once he surged in the polls and became a threat to Romney, he was battered by millions of dollars in negative ads. His campaign went into a tailspin. Now, after weak finishes in Iowa and New Hampshire, his candidacy is on the line. He knows that a bad loss here on Saturday probably would mean the end.

That is why the former House speaker grabbed for the debate lifeline again in what may have been one of the last chances for anyone to slow Romney’s momentum. Although he was the recipient of some tough questioning, he emerged as the favorite of the raucous audience in Myrtle Beach, winning repeated applause for his answers.

Had Romney lost the Iowa caucuses or turned in a less impressive victory in the New Hampshire primary, the dynamic of this race might have been far different. That is the problem that all of Romney’s rivals must contend with in the final days here, most notably Gingrich. The former speaker has many opponents this week, including himself and his own history.

Romney remains his main target, but former senator Rick Santorum (Pa.), who nearly defeated Romney in Iowa, is his major nemesis. Both men yearn to be the conservative alternative to Romney. Santorum has not taken much advantage of what he did in Iowa or of the apparent support he received last weekend from socially conservative leaders.

On Monday night, he did not shine. At one point, he and Romney ended up in an argument about voting rights for violent felons. Santorum put Romney on the defensive, but the topic was one that is not likely to win Santorum many votes among conservative Republicans this weekend.

If Santorum was lackluster on Monday, Gingrich was not. He once again hectored Romney about his record in the private sector and creating jobs as governor of Massachusetts. He defended himself against those who have criticized him for attacking capitalism in an effort to stop Romney.

Gingrich said it is important for the GOP to test a prospective nominee before Democrats get their hands on him. “We need to satisfy the country that whoever we nominate has a record that can stand up to Barack Obama in a very effective way,” he said.

Gingrich dismissed a question about whether calling Obama the “food stamp president,” a staple of his rhetoric for months, carried racial overtones. He challenged criticism of a controversial statement earlier in the campaign in which he proposed having children in poor neighborhoods work as school janitors. Critics have called him heartless. Denouncing political correctness, Gingrich brought the conservative audience to its feet cheering in his defense.

For much of the night, Romney was far more careful, even tentative. Challenged by Texas Gov. Rick Perry to release his tax returns, he ducked the question. Pressed again later in the debate, he equivocated but left the impression that he knows he cannot win the battle of public relations on the issue and probably will make the records public during tax season in April. It was a grudging response rather than a confident statement by a confident candidate.

Romney was more effective toward the end of the debate when he parried Gingrich on the issue of negative and inaccurate ads being aired by super PACs supporting each of their candidacies. And more than the other candidates, he tried to project a general-election message by turning his responses into critiques of Obama’s record.

Romney has been marching steadily toward the nomination, but in neither Monday’s debate nor the last one, before the New Hampshire primary, has he shown the kind of easy command that marked most of the early forums. That’s partly because his rivals never criticized him more directly than they have in the past two weeks. The attacks must have been anticipated, but Romney appeared less comfortable with handling them.

Romney is comforted, however, by the fact that Gingrich and Santorum continue to split the vote of enough conservatives to make it possible for him to win on Saturday with a percentage as low as that of Sen. John McCain (Ariz.) four years ago (33 percent). Adding to Romney’s advantage is that two other candidates are taking some of that vote as well.

One is the badly damaged Perry, who nonetheless turned in one of his most forceful debate performances. Under other circumstances, that might have boosted his candidacy. The other Texan in the race, Rep. Ron Paul, had a very rocky night, with convoluted answers on defense spending and the killing of Osama bin Laden that no doubt put him at odds with a majority of South Carolina Republicans.

The pattern in the GOP debates has been for Romney’s rivals to go after him, but never enough to do real damage. That may be how voters judged Monday’s session. But it is curious that the front-runner is giving some of his least impressive performances at the moment he could be wrapping up the contest.

Gingrich and Santorum will have another opportunity on Thursday in the final debate before the primary. But the forums that did so much to shape the campaign earlier might be of diminishing impact now. Too many other dynamics — some created by the disciplined Romney campaign team, some as a result of his rivals’ weaknesses — now control the contest. For that, the former Massachusetts governor must be thankful.