Santorum’s team is defying criticism of its guerrilla approach, forging ahead into the next phase of the primary race without a major adjustment in the improvisational strategy that’s been driven so far by the candidate and a clique of close advisers.

Santorum has battled concerns about his field and turnout machinery, suffering from a series of stumbles that left him without access to the Virginia ballot and ineligible for some delegates in Ohio. He leans on local political networks, powered in many cases by grass-roots Christian conservatives, as a substitute for Mitt Romney’s bulked-up organization. The campaign maintains the tightest of inner circles, reserving Santorum’s ear for a small list of longtime aides and supporters.

Of all the candidates who have sought the Republican presidential nomination, none — except, perhaps, Ron Paul — has kept his team of top strategists as insular as Santorum. Even as his campaign has added a coalitions director, digital staff and communications aides, the core of Santorum’s team is much the same as it was six months ago.

Actually, it would be only a mild exaggeration to say it’s the same as it was six years ago, when Santorum was taking advice in the Senate from GOP media consultant John Brabender and chief of staff Mark Rodgers. Both men are intimately involved in his presidential campaign, with only a handful of aides who joined Santorum early in the Iowa and New Hampshire phase of the race sharing the candidate’s attention.

And now, with the race playing out on a national scale, there’s little sign that the group intends to grow beyond the gang that knows Santorum best and with whom the candidate is most at ease.

“What you see now is what we’ve seen [from Santorum] in Pennsylvania — smart, articulate, disciplined. I think the inner circle is very, very small,” said former Pennsylvania Gov. Tom Ridge, who served in Congress with Santorum in the 1990s. “It’s always been that way for Rick.”

Chuck Laudner, a former Iowa GOP official who helped steer Santorum’s effort in the state, said the campaign is “mostly the same group” as it was there.

“There’s no entourage with this candidate. It’s a pretty tight-knit group, and they all control their own little corner of the world and take care of their responsibilities,” said Laudner, explaining there’s a “comfort level” with that small crew. “That’s part of the appeal. It isn’t some deliberate show – or lack of show.”

For an extreme underdog like Santorum, there can be advantages to maintaining a slender campaign. By not expanding his circle, Santorum has kept himself largely insulated from some of the pressure others might feel in making a serious push to topple Romney. Multiple aides emphasized to POLITICO that there’s a “nimbleness” to Santorum’s campaign that lets him adjust to changing circumstances without overcommitting to any one set of tactics.

Brabender, who essentially launched his career as a political consultant with Santorum’s 1990 House race, described the setup as an invention of himself and Rodgers aimed at playing to the candidate’s personal strengths and tapping into the advent of the decentralized, cause-driven, low-cost modern campaign.

The Santorum camp has an office in Virginia, but on daily senior staff calls, Brabender said participants are often spread across eight or nine states. The GOP ad man said he speaks almost hourly about the campaign’s fundraising with finance director Nadine Maenza, who was a top fundraising aide for Santorum’s 2006 campaign. Campaign manager Mike Biundo is typically in the field — in a targeted state like Ohio — rather than running the trains at headquarters.

For both Santorum and his advisers, the live off the land-style outfit is entirely familiar, Brabender said.

“In 1990, I didn’t have a clue and [Santorum] didn’t have a clue. And in some ways it was a great advantage, because we weren’t supposed to win,” he said. “We didn’t know the rules, and so we made up new ones.”

Brabender added: “I think there’s some of that with this campaign, quite frankly, too.”

The touch-and-go quality to the campaign, which many find endearing, also leaves the candidate exceptionally vulnerable to his own shortcomings. Around Santorum, there’s no protective layer that can stop him from spouting off on, say, John F. Kennedy’s view of church and state. There are few aides who can nudge him toward a more disciplined, sustained set of talking points. With few surrogates and senior aides at his disposal to hold press calls and hit the cable circuit, Santorum has found himself consistently outgunned by Romney in the messaging department.

Santorum’s campaign, said one Republican familiar with his operation, has “never shown an ability to drop a bomb and drive a story.”

“I almost feel like with [Santorum], he’s their only messenger. … They need to add more brains to the operation and more high-level people — people that have run a U.S. Senate campaign, people who have worked at one of the party committees,” the Republican said. “Feb. 7 [when Santorum won three primaries and caucuses] provided them an opportunity to address some of these problems. Right now, this period in the race does the same thing.”

There’s a blueprint for exactly that kind of expansion. In the 2008 cycle, Mike Huckabee padded his upstart bid as soon as it began to gain ground, bringing on veteran Republican strategist Ed Rollins as his campaign chairman and recruiting former Reagan and George H.W. Bush aide Jim Pinkerton as a senior adviser.

Santorum, on the other hand, has added to his staff only in the most instrumental of ways, hiring a new national press secretary, a delegate counter and filling a number of other operational posts.

Part of that, said Santorum communications director Hogan Gidley, is simply a matter of the candidate’s preferences. When it comes to the counsel he takes, Santorum evidently sticks to a familiar set of voices.

“The campaign mirrors the candidate, and Rick is very well-organized, but he doesn’t have a big circle. He has a tight circle,” Gidley said. “He’s a good manager because he’s not a micro-manager. But we do run things by him. It’s a small enough staff that we can do this.”

The fact that the campaign is now playing out across 50 states, rather than the three early battlegrounds Santorum’s effort was built for, is a double-edged challenge.

On one level, Romney’s structural and financial advantages loom ever larger as the race moves into places where the GOP hopefuls cannot run a months-long, hand-shaking, Santorum-style retail campaign. On the other hand, it means that national-level messaging can have a disproportionate impact in states like Alabama and Mississippi, two states voting on Tuesday in which no candidate has been able to build up a daunting organizational edge.

“I think the ground game is important in a lot of ways,” said Alabama Rep. Mo Brooks, who is neutral in the presidential primary. “But no candidate has an overwhelming ground game or could be expected to have an overwhelming ground game, given their limited time, their limited resources and the fact that they’re covering the entire United States.”

In the department of day-to-day, hour-to-hour communications and strategy, however, Santorum’s loyal band still has a lot to prove, and the cost of a hundred small missed opportunities is clearly mounting. Unaligned strategists in both parties shook their heads in disbelief Thursday after Santorum’s campaign failed to answer a Romney charge criticizing him for his ties to the K Street lobbying world.

Romney’s campaign has extensive ties to lobbyists and former lobbyists, and, only hours before, Romney had announced the support of ex-Missouri Sen. Kit Bond, who quickly joined the well-connected law firm Thompson Coburn after retiring from the Senate.

Santorum’s campaign didn’t point out any of that. To the dismay of Romney’s foes, they just took the hit.

“Every day we track as the Romney campaign touts endorsement after endorsement from lobbyists. So for the Santorum campaign to not fight back and show that Romney’s attacks are totally hypocritical is asinine,” said Ty Matsdorf, senior adviser at the oppo-slinging Democratic independent group American Bridge. “If it was malpractice for Gingrich to take four months to fight back against Romney’s Fannie/Freddie attacks, this is even worse because given how weak Romney is, with a little bit better of an organization Santorum would be cruising to the nomination.”

Far from worrying about the cost of such missed opportunities, Santorum advisers say they’re confident in their ability to strike back when it counts, and continue to drive a larger contrast message about which candidate is more in sync with the heart of the Republican Party.

“We have rapid response when we need it. … We put out our own things, too. What we’re not going to do is just get into reactive mode, either,” Brabender said, asking of Romney: “Do they have the largest negative mud machine in the history of presidential campaigns? Absolutely, and if that’s what it takes to get elected, then we should just declare it’s over right now.”

Suggesting that campaigns reach a point of “diminishing returns by just adding people,” Brabender argued that the skeptics are underestimating what Santorum’s team has built.

“Is it Mitt Romney’s sort of bloated, establishment, federal government-style campaign? No. But part of that is by design,” Brabender said. “I think probably if you compared us to Gingrich’s campaign, I think we’re probably much more sophisticated.”