Mitt Romney speaks at a town hall meeting in The Villages, Fla. (Reinhold Matay/AP)

All along, everything has gone according to Mitt Romney’s plan. His strategists didn’t believe that Tim Pawlenty would catch on. They were confident that Michele Bachmann would fade. They were prepared for Rick Perry. They never thought Herman Cain would pass the commander in chief test.

But they didn’t count on a late and strong rise by Newt Gingrich.

Once left for dead, the former House speaker has suddenly emerged as Romney’s most durable opponent yet — in part because he has performed well in the debates and, unlike the others, he is viewed by many in the Republican Party as a plausible president.

For this unexpected turn in what has been a steady and sure campaign, the Romney team has no road map. With just five weeks until the Iowa caucuses, the former Massachusetts governor and his advisers are trying to figure out what to do. Will they stick to their tried-and-true playbook and hope Gingrich falls on his own, just like the others? Or will Romney engage Gingrich directly and aggressively, either through ads or in a pair of upcoming debates?

“Is there enough time for Gingrich to self-destruct on his own before Jan. 3, or do you have to help it along? It’s a tough call,” said a GOP strategist who informally advises Romney’s campaign and, like other advisers interviewed, spoke on the condition of anonymity to discuss internal thinking.

Romney’s strategists are gaming out scenarios. They say they understand the risk that, in a multi-candidate field, any attack they make against Gingrich could boomerang to hurt Romney and help a third candidate.

Taking on Gingrich is “going to be a process,” one adviser said. “It’s not going to be an overnight kind of a thing, unless he steps in it. But he seems less likely than the others to do that.”

At a time when Romney intended to be showing momentum and closing the deal with voters, his campaign has sometimes been on the defensive. The candidate appeared rattled in a Fox News interview Tuesday when he was pressed by host Bret Baier to explain his changing positions on some issues.

“Bret, I don’t know how many hundred times I’ve said this, too. This is an unusual interview,” a visibly agitated Romney said, as he wiggled in his seat, crossed his legs and forced a laugh. “Ha, ha, ha, ha. Let’s do it again.”

In that same interview, Romney hinted that he now sees Gingrich as a threat. Prompted by Baier, he launched his first attack on his rival, labeling Gingrich “a lifelong politician” and suggesting that he lacks credibility on the economy.

The informal Romney adviser said he recently urged top campaign aides to take a more aggressive posture, telling them: “You’ve got to get out and fight for it. The shot’s been fired, and you’ve got to go.”

A campaign spokeswoman declined to comment.

Across the country, many of Romney’s donors and political supporters said there is no sense of panic over Gingrich. Romney’s network takes comfort in the great financial and organizational advantages that he has amassed to help him survive a potentially grueling nomination fight.

“Newt’s having his moment, but I think it’s nothing more than that,” said New York Jets owner Woody Johnson, Romney’s national finance co-chairman, who added that he knows and respects Gingrich.

Johnson said he is confident in Romney’s strategy and has been impressed by his performance. “Romney has been very restrained and focused really on Obama and the task at hand rather than the other candidates,” he said.

Florida state Sen. John Thrasher, a member of Romney’s advisory committee, said he also feels confident in the candidate’s structural advantages, but he thinks the campaign needs to respond to Gingrich’s rise with vigor.

“There’s going to be some need to explain his positions,” Thrasher said, noting that Gingrich’s past views on global warming “won’t sit well with the base.”

This is a pivotal time for Romney, whose campaign has projected a sense of inevitability even as he has struggled to build on his support. Now begins an intense period of testing for the candidate, who will have to win over some skeptical voters if Gingrich succeeds in consolidating support as the conservative alternative in an ostensible two-man race.

“His chief opponent is now known,” said GOP strategist Steve Schmidt, who managed the campaign of 2008 nominee John McCain. “And his chief opponent is an extremely gifted communicator who’s rising on the basis of his incredibly strong debate performances, who has become legitimized as an aspirant for president of the United States.”

Adding to Romney’s challenge are the attacks from President Obama’s campaign and the Democratic Party, attacks designed in part to sow doubts about Romney’s consistency and conservatism among GOP primary voters.

“There’s an impact from that. That’s why we’re concerned, that’s why we’re responding and we’re doing a counteroffensive,” said Ohio state Rep. Jay Hottinger, one of Romney’s top surrogates there, who is representing Romney in a televised debate Thursday against former Ohio governor Ted Strickland, an Obama surrogate.

Hottinger said he is encouraged by Romney’s solid core support in state and national polls, but he added that “from a discouraging standpoint, he just doesn’t seem able to catch on with a certain element within the party.”

On the campaign trail in recent weeks, Romney has picked up a parade of endorsements from prominent office-holders in the early-voting states. But his campaign has struggled to show enthusiasm. While Gingrich has been drawing boisterous, overflowing crowds, Romney’s events in Miami and Tampa on Tuesday drew smaller and more subdued audiences.

A Gallup poll released Tuesday showed Gingrich’s “positive intensity score” surging among Republicans as Romney’s score dropped to his lowest level of the year. Gingrich’s score, however, has not reached the levels achieved earlier by Cain, Bachmann and Perry before they quickly fell from front-runner status.

The question looming over the race is whether Gingrich will suffer the same fate. Increasingly, Romney’s advisers believe he will not. They are calculating that primary voters will overlook Gingrich’s rocky career in public life — including ethics charges, extramarital affairs and a decade of trading his influence to enrich himself.

Voters “know all that, and they’ve discounted those things,” said a third Romney adviser. “The leadership of the campaign recognizes that Gingrich has got staying power, that he’s a very serious candidate.”

This adviser said that the campaign considers Gingrich’s rise “absolutely” different from Cain’s or Perry’s, in part because they cannot count on him withering under his new spotlight.

“Clearly there is a very strong, clear view that it’s a real contest,” the adviser said. “But Romney is doing the right things to win it. It’s not slipping away. Romney’s got to fight for it. He always was going to have to fight for it. It’s just now clear who the fight is with.”