By Kim Dixon

WASHINGTON (Reuters) - Surging Republican presidential hopeful Herman Cain's fund-raising figures are expected to rise when he releases his third quarter report this week, but analysts say he faces big hurdles in translating that to a winning national campaign.

Cain has jumped to the top of some opinion polls among Republicans looking to unseat Democratic President Barack Obama since a surprise win in a straw poll last month and stronger debate performances.

He was second this week in a Reuters Ipsos poll and first in an NBC-Wall Street Journal poll.

The former pizza executive, who has never held elective office, raised just $2 million in the second quarter, among the lowest of the major presidential aspirants. His newfound popularity and more media coverage will likely lead to a jump in donations in the third quarter, experts said.

"Certainly with his higher media profile as well as his higher profile in the debates, he should see a bump in smaller contributions and maybe mid-sized contributions," said Eric Ostermeier, a research associate who runs a popular politics blog for at the University of Minnesota.

But experts say viable candidates need a mix of big and small donors to sustain a national campaign. Big donors provide the money to keep a campaign running and small ones are essential because they provide repeated donations over the long haul.

Under U.S. election laws, individuals can only give a total of $5,000 directly to each candidate -- split between the primary and the general election.

Cain's second-quarter report showed 54 percent of his donations coming in chunks of $200 or less, according to an analysis by the Campaign Finance Institute. Just 3 percent of his donors maxed out at the $2,500 giving level.

STAYING POWER?

It remains to be seen whether Cain can attract the big donors and whether he is building any type of national organization.

"I'll believe he has staying power when I see it," said Dave Peterson, an Iowa State University political science professor. "National polls are worthless at this point when 48 states are paying no attention to the campaign at all and the two that are (IA and NH) aren't really that focused yet either."

Peterson said Cain lacks what other candidates have -- Mitt Romney's and Rick Perry's big players in the Republican establishment and a highly committed base of support, as Ron Paul has.

Cain's profile has steadily risen in the past month. His time speaking in debates has risen from about 8 percent -- the lowest of all candidates in the first Florida debate -- to about 12.5 percent in the New Hampshire debate this week, according to an analysis by Ostermeier.

Cain also runs a political action committee, Hermanator PAC, which raised less than $220,000 in 2010 and has raised just $21,000 so far this election cycle, according to the Center for Responsive Politics.

(Editing by Philip Barbara)