Mitt Romney is running out of rich people.

The former Massachusetts governor’s gold-plated fundraising machine has depended almost exclusively on big checks from the wealthy individuals — business leaders, Mormon allies, longtime Republican Establishment donors — whom Romney and his team have been cultivating for years, pressing the cases that resonate for those crowds: That he’ll cut taxes; that he’ll beat Barack Obama; that he’s inevitable and they’d better get on board.

But Romney has proved unable to tap into the emotion-driven small-dollar contributions that helped power Barack Obama in 2008, and which fueled even his more Establishment rival, Hillary Clinton, this time four years ago when she too began to run out of big donors. The result: Republican fundraisers say that despite his success so far, they think Romney is fast approaching a wall, and that he will likely be forced to pay for the campaign out of his own deep pockets.

“I don't know that he’s completely tapped out, but they are trying to look under every stone,” said a top New York Republican fundraiser. “You run out of people you can hit up for $2,500,” he said, referring to the legal limit for primary contributions.

“Here in New York he has had fundraiser after fundraiser after fundraiser,” said another top New York Republican. “Even in New York, there’s only so much you can get out of the city.”

Romney is unusually dependent on those contributions. Over 82 percent of Romney's haul from individuals in 2011 came in donations greater than $1000 —which doesn’t even include the larger donations to the SuperPAC that supports him, Restore Our Future. And as Rick Santorum stays in the game with a growing pool of small donors — to whom he can return for another $20 or $100 when the millions they’ve given him runs out — Romney’s campaign has launched an intense, late scramble for the cash to fend off his conservative challenger.

A Romney aide pointed to a new media buy in Michigan (valued at $1.3 million by a tracker) as evidence that the campaign continues to raise money successfully, and another insider said a push for cash this week had been a success. Romney raised about $1 million at a Washington, D.C.-area fundraiser tied to a “policy roundtable” last Thursday, and he picked up another $750,000 at a single Los Angeles event Monday, fueled in part by bundlers who joined his team from Rick Perry’s defunct campaign.

“It’s been a huge couple of days,” the source said. “People realize that Santorum has no chance in the general, and some of the money folks on the sidelines are starting to jump in now.”

Romney also has a “young professionals” fundraiser planned for next month in New York, hosted by the hedge fund manager and former Obama backer Dan Loeb.

But while Romney’s camp continues to project optimism, Republican fundraisers who are less closely involved in the campaign say the model simply isn’t built for a long battle with Santorum.

The fact that fundraising is “slowing down” has has become a recurring topic of conversation among Romney bundlers, a third Republican fundraiser said, providing a bullet-pointed list of six reasons for the difficulty:

–Romney has been “tapping out” major cities including New York, Miami, Boston, and Los Angeles.

–Romney has less time to raise money because he’s been on the road in places like Missouri and Maine, where there’s little money to be found. “Not a lot of money comes out of those states,” the fundraiser said. “He now only has time to pop into major fundraising cities for just a few hours.”

–Romney doesn’t inspire much more enthusiasm among the rich than he does small donors: “Romney is still presumed the front runner and donors think he is going to win, but the excitement level is still very low.”

–The Romney campaign narrative undercuts low-dollar fundraising: “With Romney the presumed ‘front-runner’ he is expected to win every state. So when he does win, it is not shocking, but expected; and when he losses, it is a major disappointment. Neither are good recipes for low dollar fundraising.”

–The donors weren’t ready for an intramural dogfight: “They never expected this Santorum surge and quite frankly aren't comfortable calling their colleagues with the pitch ‘help Mitt Romney defeat Rick Santorum in the primary.’ It is honestly a little embarrassing. They don't understand how the campaign hasn't already locked up the nomination. Donors see it as a waste of money and don't take the Santorum surge seriously."

–For the ultra-rich, It’s easier to write a big check to the Super PAC than to pester your friends for $2,500 contributions to the campaign. “What you are seeing happen with the Romney camp now is that these huge donors have two options: they can either spend hours a day on the phone bundling $2,500 checks from friends and family, or they can just drop $1,000,000 into the Romney Super PAC and feel that they have done their ‘duty.’”

The last feature, of course, means Romney isn’t going to be out of money. As long as he can keep paying his relatively modest staff — he has, a supporter noted, a more streamlined campaign and fewer “hangers on” than in 2008 — and travel bills, Restore Our Future can provide the air cover.

But the campaign remains dogged by the sense that donors are beginning to give up. Three Republicans who work closely with Romney’s bundlers said they had begun to shift their focus away from the presidential campaign entirely.

"All the low-hanging fruit has been picked," said a top New York finance industry donor and fundraiser who is currently neutral in the race and described Romney as "a Republican John Kerry running Hillary Clinton's campaign."

“The view is: ‘I’m focusing on the senate races now and that’s it,’” said the New York Republican. “There’s a shift here, and it’s driven by the amount of money he had to spend to pound each of his adversaries."

“You need a positive message to win, and the economy‘s coming back a little bit and Obama’s up a little bit , so they’re all feeling, ‘Let’s focus on the Senate.”