WASHINGTON (AP) — As they try to derail Mitt Romney's presidential campaign, his challengers face an urgent task: raising enough cash to compete.

The campaign for the GOP nomination is moving into states that are more dependent on TV advertising, and nobody has yet been able to match the former Massachusetts governor's financial operation.

Rick Santorum, riding a wave of momentum, nearly won Iowa's caucuses on a shoestring but has no national fundraising network. Newt Gingrich vows to go after Romney in New Hampshire, but if history is any guide, he lacks the cash to do it. And Texas Gov. Rick Perry, a prolific fundraiser, still might be a threat — but the Iowa results showed his support has waned.

Romney's campaign has spent more than $17 million since January 2011, while Gingrich and Santorum have each spent less than one-sixth of that.

Santorum, who finished just eight votes behind Romney in Tuesday's caucuses, has already seen a surge of online donations, enough to cripple his campaign's website shortly after the Iowa results were announced.

Santorum aides reported raising $1 million Wednesday alone, and campaign manager Mike Biundo has said the campaign's fundraising pace has tripled over the last week.

But with just five days to go before the New Hampshire primary, it's unclear how Santorum, Gingrich and Perry can compete financially with Romney in New England and beyond.

An Associated Press review of federal campaign-finance data reveals a stark contrast between Romney and most of the GOP's remaining candidates. Romney has a national donor network that's raised more than $32 million during the first nine months of 2011, the most recent data available.

Other candidates are more limited. Santorum received many of his contributions from Pennsylvania and Florida. Much of Perry's money — including from outside groups — came from Texas. Ron Paul, with $12 million, has been financially competitive, but his libertarian views limit his appeal to GOP primary voters.

Romney had little trouble raising cash from across the nation during that same period — garnering donations from liberal-leaning districts as well as GOP strongholds in the South.

Complicating this election has been the explosion of outside political action committees, known as super PACs, which have spent millions in support of their favored candidates. While they must legally remain independent from candidates, they've helped the Iowa front-runners in part by deflating the brief surge of Gingrich, who especially suffered from negative ads by the Romney-backing Restore Our Future PAC.

Among the disparities so far:

—Individual donors: To date, Romney has drawn more than $32 million in individual contributions. Gingrich took in just under $3 million since early 2011, Santorum just over $1 million.

—Super PACs: These outside groups spent at least $5 million on ads leading up to the Iowa caucuses, notably from Restore Our Future. Santorum's campaign spent an anemic $4,200 on ads in the state, while his supportive super PAC — the Red, White and Blue Fund — spent about half a million dollars in Iowa media markets and is expected to spend more in other primaries.

—Defunct campaigns: Candidates often seek to recruit the fundraisers of their rivals who withdraw from the race. But following Tuesday's results, only one candidate — Rep. Michele Bachmann — has dropped out. Since January, the Minnesota congresswoman has raised about $5 million but reported about $1 million cash-on-hand this fall, putting her near the middle of the GOP fundraising heap.

As Santorum and the others rush to expand their operations, Romney is wasting no time. On Wednesday morning, his campaign released a new campaign video, titled "American Optimism," which makes a nod toward New Hampshire.