The role reversal that took place during Monday’s Republican presidential debate proved two things: Mitt Romney knows he desperately needs to win the Florida primary on Tuesday, and Newt Gingrich isn’t the same candidate onstage without a boisterous audience backing him.

After the shellacking Romney received in South Carolina, his front-runner’s aura disappeared. No more trying to glide through a debate against rivals he regarded as minor threats or political lightweights. For the first time, he met his leading rival as an equal or maybe even an underdog.

Voters in Florida will reveal just how weakened a candidate Romney has become. One loss to Gingrich, in the South Carolina primary on Saturday, was a setback. A second could be crippling. Romney’s sudden pivot since that defeat shows just how much he understands that he has little time to reverse Gingrich’s momentum and reestablish himself as the front-runner for the nomination.

The past two days have encapsulated everything about Romney’s hurry-up strategy. A candidate who has avoided Sunday talk shows appeared on “Fox News Sunday” the morning after his double-digit loss and said that he would release his tax returns. He acknowledged that he should have done so earlier. He deserves no praise for being blind to the political realities for weeks, nor for finally recognizing the obvious.

Romney arrived in Florida later Sunday with rhetoric sharpened by defeat. He went after Gingrich with tough new language, although the video of the debate shows a candidate who does not take easily to the role of attack dog. He is too gentlemanly for that.

On Monday morning, Romney hurled the kitchen sink at his rival. The across-the-board attacks were an effort to put everything negative about Gingrich into the discussion as the voters in Florida began to pay close attention.

Romney challenged Gingrich over his tenure as House speaker, calling it a failure. He reminded voters that his rival had been reprimanded by his House colleagues for ethics transgressions. He raised questions about Gingrich’s work for Freddie Mac and about his dozen or so years as a Washington consultant who made himself wealthy by, as Romney put it, peddling influence.

It all came together on Monday night in the first third of the 90-minute debate sponsored by NBC News with the National Journal and the Tampa Bay Times. NBC anchor Brian Williams barely had to prod Romney to go on the attack. At every turn, with every question, after every Gingrich response, Romney pressed his case relentlessly.

There have been flashes of this Romney in the past, most notably when Texas Gov. Rick Perry first appeared at a debate as the rising candidate in the race. That was at the Ronald Reagan presidential library in California in September, and Romney came prepared to challenge on Social Security the man his campaign feared. The difference on Monday night was that Romney realized that his candidacy is truly at risk.

He has proved peevish when attacked in debates, impatient with his rivals and, at times, looking to the moderator for help to shut down an uncomfortable encounter. On Monday night, he sought to avoid being defensive by staying on the offense through critical exchanges. With every Gingrich parry came a Romney riposte.

Romney’s case was not built on a foundation of superior ideas and truer conservative convictions. Instead, his strategy to overcome the doubts about his weaknesses appeared to be no more complicated than simply to try to discredit Gingrich in voters’ eyes.

His candidacy has been staked on the hope that Republican voters will buy his argument that because he was a successful businessman, he offers the best contrast to the president on the top issue facing the nation: the economy. What he truly believes — what his core convictions are — has been a hole in his campaign. That is why Monday’s debate may have been helpful to him but not dispositive. Republicans may be looking to see more fight from Romney, and they may continue to question who he is.

Gingrich has convinced many Republicans that he is best equipped to take on President Obama in the fall debates. He has been skillful in leveraging the energy of debate audiences to amplify his critiques — of the president, of the news media, of liberal academics and of everything else that the Republican base loathes. That was the essence of his South Carolina campaign.

But Monday’s forum was played under rules closer to those that will prevail during the general-election debates. Those rules produce a far more staid environment. No cheering, no booing, no standing ovations. The audiences are implored to be silent. Persuading swing voters is often the goal in such forums, not feeding the passions of the base.

On Monday, Gingrich was far more restrained than he usually is in debates. Whether that will cause GOP voters to reevaluate him is questionable. Thursday’s debate in Jacksonville will return to the charged environment of most of the previous exchanges in this contest, to the delight of partisans who found Monday’s forum — at least after the first 30 minutes or so — on the edge of boring.

Gingrich competes best from behind. He feeds off grievance. As the surging candidate coming out of South Carolina, he is being forced to run as, if not the leader of the pack, then the man to beat at the moment. He is trying to project himself as reasonable, presidential even, knowing the consequences if he slips into the “bad Newt” who has been familiar over the past three decades.

His speakership will always be rife for criticism, given the turmoil that followed in his wake during those four turbulent years. He has made the case that, overall, he has done more to build the Republican Party than Romney has — and Romney cannot refute that. He must hope that Republican voters will forgive him his excesses and look to the other side of the ledger.

Gingrich’s post-Congress work is another matter. He was most aggrieved Monday night when Romney accused him of a conflict of interest over his advocacy for the prescription drug benefit added to Medicare. In Gingrich’s telling, what he did was what he has always done: advocate publicly for the ideas he champions. He is, he said, proud of what he did to expand Medicare coverage.

Romney’s counter was that Gingrich was making that case publicly and to members of Congress at a time when one of his companies was receiving money from health-care firms that stood to benefit from the drug benefit.

Gingrich considers himself above such conflicts. He believes he transcends such transactions. Romney is likely to prod further on whether his rival crossed a line, at least in principle, just as he will press the case that Gingrich benefited financially from his work for Freddie Mac at a time when that agency was contributing to the housing foreclosure crisis that hit Florida’s economy hard.

The next 24 hours of the contest will be dominated by the president and his State of the Union address. When Republican voters next see their presidential candidates onstage at Thursday’s debate, they will have fresh images of candidate Obama etched into their consciousness. They are still looking for someone who will give them confidence that the party can win the White House in November. Monday’s debate did not provide the answer.