Carson spokesman Doug Watts says the campaign is not worried about the burn rate.

“It’s not only sustainable, it’s strategic and it’s profitable,” he says. “We have a more innovative approach here. We’re using modern tools that allow us to do things more efficiently. We’re doing a lot of things with tools that are not on most people’s radars. They’re showing effective results in these polls.”

But some of Carson’s most-favored tools are among the oldest in the book. His campaign relies on direct-mail and telephone fundraising—literally sending fliers to voters or cold-calling them and trying to talk them into giving money. Those are common tactics for political campaigns of all types, and in particular for Republicans, since they reach older voters better. One advantage these methods confer is that they help to build up a grassroots base. Carson has astonishing grassroots support, with a wide base of small-donor dollars, and that support has helped push him near the top of the Republican field.

“Strategically it was our plan to build a donor base,” Watts said. “There’s only one way you do that—you invest in prospecting in direct mail, prospecting in telemarketing, prospecting online. By the end of the year, we’ll have a unique donor file of half a million names.”

The downside of the tactic is that it costs a lot. “Prospecting is expensive,” Watts added. You’ve got to mail the fliers—nearly $2.5 million, or about a quarter, of what Carson spent in the third quarter was categorized as postage-related—and you’ve got to have human beings making those telemarketing calls. And even when the investment pays off, what it yields are small-dollar donations—$25, $50, $100—rather than anything approaching the $2,700 maximum.

While direct mail has its detractors, even its champions argue that the goal is to spend less on it over the course of a campaign—to make direct mail obsolete. As a campaign builds its donor file, it can return to those supporters repeatedly, convincing them to give a little more each time. Carson’s campaign burn rate actually increased in the third quarter, from 64 percent in the second quarter, but Watts said the fundraising costs are down.

“We spent 54 cents on the dollar to raise money this quarter,” he said. “Last quarter we spent 64 cents on the dollar. That is an 11 cent reduction. I expect we’re probably going to have a 10 or 11 cent or maybe more reduction the next quarter.”

The campaign has some 700,000 donors, about 300,000 of whom have given more than once, and Watts said several thousand donors moved from giving less than $200 to giving more than that amount in the third quarter. The test for the Carson campaign’s strategy will be to see whether the portion of repeat givers continues to rise, whether their donations continue to climb, and whether the amount spent to raise each dollar continues to fall.