Republican presidential candidate Marco Rubio speaks during a campaign rally at the Utah State Fairpark Monday, Oct. 19, 2015, in Salt Lake City. (Rick Bowmer/AP)

Marco Rubio has been inching up in the Republican presidential primary contest for weeks, boosted by a pair of strong debate performances highlighting his raw political talent and potential.

But a modest fundraising haul last quarter, along with his relatively minimal campaign structure in the early voting states, underscore how much ground the freshman senator from Florida must gain over the next three months before the first nominating contests begin.

Rubio has also lagged behind tea party favorites Ted Cruz and Ben Carson in collecting small donations from grass-roots supporters. Rubio has a more bare-bones campaign operation than GOP establishment rival Jeb Bush in the first four nominating states.

Rubio’s $5.7 million haul during the third quarter of the year was less than half of the totals put up by Bush, Cruz and Carson, even as he focused intensely on raising money. Since announcing his White House bid, Rubio has spent nearly $2 million on fundraising, accounting for 26 percent of his total spending. Of that total, $820,000 came in the third quarter.

The Florida senator’s campaign believes its slim operation will allow him to survive a crowded field in which campaign donations are being spread thin and that his talent as a candidate will elevate him in the long run. But other Republicans are not so sure.

Sen. Marco Rubio, R-Fla., gives a campaign speech during a stop at BOC Water Hydraulics on Friday, Oct. 16, 2015, in Salem, Ohio. (Scott R. Galvin/AP)

“His play-it-safe, peak-at-the-right-moment-but-not-too-soon strategy also means he’s not deeply resonating with a segment of the electorate,” said former New Hampshire Republican Party chairman Fergus Cullen, who has not endorsed a candidate.

Since starting to raise money for his presidential bid, Rubio has collected 22 percent of his haul from donors giving $200 or less, according to a Washington Post analysis. That compares with 41 percent for Cruz, a Texas senator, and 62 percent for Carson, a retired neurosurgeon.

Bush lags behind all of them with 5 percent. But the former Florida governor has established a deep organization in the first four nominating states, including a dozen paid staffers in New Hampshire, 10 in Iowa, eight in Nevada and seven in South Carolina, according to the Bush campaign. He has seven campaign offices spread across the four states.

“They are limited because of their candidate,” former Iowa Republican Party political director Craig Robinson, who is neutral, said of Bush’s team. “Rubio is the exact opposite — a superior candidate and a very weak campaign apparatus.”

The Rubio campaign declined to provide a state-by-state breakdown of staff and offices after initially suggesting it might do so. It said it has an office in each of the early states, with “multiple” offices in some. It said that about half of its 54 full-time staffers are in the four early states, divided fairly evenly among them.

“One of the biggest mistakes you can make in a presidential primary like this is to mistake action for progress,” said Rubio deputy campaign manager Rich Beeson. He later added: “The days of having to have 50 field staffers and 25 offices are done. We can have a field office and staff set up in a Starbucks with wireless and get just as much done as we can in a brick-and-mortar office with land lines.”

Republican Presidential candidiate Marco Rubio is introduced at the Republican Committee of Chester County Fall Reception at the Crowne Plaza in King of Prussia, Pa., Thursday, Oct. 15, 2015. (Elizabeth Robertson /AP)

Wisconsin Gov. Scott Walker illustrated the perils of expanding a campaign footprint too quickly. He raised a healthy $7.4 million in the third quarter but raced through nearly all of it on a payroll of more than 80, generous paychecks for top staffers, dozens of consultants and elaborately staged campaign events. Walker dropped out last month, telling supporters that he did not have enough money to continue.

Polls show that Rubio and Bush are running about even in the early states. But both are trailing front-runner Donald Trump considerably.

After his strong performance in the second debate, Rubio has been gaining ground among the GOP money class. During a swing last week through New York, Philadelphia and Pittsburgh, the senator raised at least $850,000 at five events, according to people familiar with the totals.

A Manhattan reception on Oct. 14 was Rubio’s most successful fundraiser to date, attended by 300 Republican donors and fundraisers packed inside a spacious Fifth Avenue law office adjacent to Central Park.

Last week, Rubio’s campaign touted new fundraising reports showing that he had about $11 million on hand as of the end of September, compared with more than $10 million for Bush. But Rubio’s total included $1.2 million that he could use only in the general election, compared with $270,000 for Bush — meaning that Bush had more money on hand that he can use now, $10 million to $9.75 million.

Rubio rarely engages directly with Bush, and his surprising decision to do so over money has put him on defense. He was repeatedly grilled about whether he was being misleading on his numbers during a Fox News interview Monday night.

Trump, the real estate mogul who has led in the polls for months, is mostly self-funding his campaign, freeing him to speak as he chooses without fear of alienating donors.

Rubio does not have this luxury and has carefully avoided taking controversial positions outside the GOP mainstream. On policy, he is closely aligned with Bush and has mostly sought to distinguish himself from Bush by running as a fresher alternative to him.

The Florida senator’s caution has limited his ability to stand out and build a loyal following in a packed and wide open field.

Cullen, who recently watched Rubio campaign in New Hampshire, said he was struck by how circumspect he sounded.

“There was no news,” he said. “It wasn’t particularly bold.”

Matea Gold contributed to this report.