Barack Obama rode to victory in 2008 on a record wave of fundraising that allowed him to drown his opponent in advertising and rack up victories far into Republican territory.

But with just 100 days until the 2012 election, President Obama faces a far more difficult financial task in his bid for reelection — battling a well-funded challenger in a narrow band of swing states, which will be inundated with attack ads and campaign visits.

Four years ago, the two presidential campaigns spent big in nearly half the country. But the fight this year is concentrated in fewer than a dozen states that are suffering through more political ads than ever before. In the pivotal swing state of Ohio, Obama has dumped $12 million on ads so far, which is four times the amount he spent at this point in 2008.

The deluge is funded not only by Obama and his Republican challenger, Mitt Romney, but by a motley and shadowy mix of outside groups, many of them backed by millionaires. The contest also marks the first time since the post-Watergate era in which neither candidate is taking advantage of public financing, which would have limited the amount of money the campaigns could spend.

The result is a crabbed contest far removed from 2008, when Obama spent relatively little time hosting fundraisers yet still managed to bring in as much as $6 million a day in the final months of the race. Obama’s figures are down this year, however, and both candidates are racing to squeeze in as many donor events as possible.

This weekend, the Obama campaign was planning thousands of events across the country aimed at mobilizing volunteers 100 days before the election.

“There’s much less room for error in 2012 than there was in 2008,” said Ken Goldstein, president of Kantar Media/Campaign Media Analysis Group, which tracks political ad spending nationwide. “I don’t think we’re going to see a world where Obama has an advantage in money.”

The incumbent is still a champion fundraiser by historical standards, particularly among grass-roots donors. But in recent months, he has fallen behind Romney and the Republican Party, which outraised Obama and the Democrats in May and June. The presumptive GOP nominee is also bolstered by well-funded super PACs and other conservative groups.

Under the gun financially and battling low approval ratings, Obama’s campaign is concentrating its advertising in nine swing states this year. That’s down from more than 15 states that it was targeting at this point in the 2008 election, when Obama was competitive in red-leaning states such as Indiana and even toyed with attempting to flip GOP strongholds such as Georgia and Montana.

Many of those targets are out of reach this year, resulting in a much heavier dose of advertising in the remaining swing states. Many voters are already turned off by the deluge, making it even harder for Obama or Romney to break through, according to experts and voter interviews.

“It’s October in July,” said Erika Franklin Fowler, director of the Wesleyan Media Project, which analyzes campaign ads. “This just hasn’t happened before. . . . In 2008, we saw a lot more markets in play than we do this cycle. There are more ads crammed into a much smaller air space.”

In Cleveland — the largest market in a state crucial to both sides — the campaigns and their allies have spent $13.6 million on broadcast television, according to data from Goldstein’s firm. In a single week this month, the campaigns spent $1.2 million to air more than 2,000 ads on Cleveland stations; that’s an average of one political spot every five minutes, 24 hours a day.

Similar volumes have inundated the rest of Ohio. “The ads are ridiculous,” said Julie Johnson, 48, a high school English teacher and Obama supporter from Perrysburg, near Toledo. “I’m very tired of the ads, and it’s only July.”

Jane Peters, owner of Media Management Services, which handles advertising purchases in the Columbus market, said her regular clients are routinely being bumped off the air by political groups that will pay top dollar for spots, particularly during news programs.

“It boggles the mind the money that’s being spent,” Peters said. “At this rate, by September and October, they’re going to take every spot that’s out there.”

Florida, home to some of the most expensive markets in the nation, has seen $30 million in spending related to the Obama-Romney race so far. In the third week of July, campaigns and outside groups spent $1.2 million on 1,741 spots in Tampa alone.

Tim Post, a 34-year-old Republican from Tampa who voted for Obama in 2008 but is undecided this year, said he’s tuning out the negative ads from both sides. “I can’t stand all the trash talking,” he said. “I’m so over it. I’ve turned a deaf ear to it. I can’t handle it.”

In New Kent County, Va., in another key battleground state, retired federal radar technician Harvey Caldwell Jr., 77, estimated seeing the same political ad half a dozen times in one evening. He and his wife, Julie, 72, say they support Obama.

“We know it’s going to get worse,” Julie Caldwell said. “We don’t like the negative stuff. . . . We made our choice already.”

The difficulty facing Obama is evident in overall ad spending. As of last week, Romney and his allies had outspent Obama and his allies by $77 million to $71 million, a reversal from four years ago when Sen. John McCain (R-Ariz.) was badly outgunned by Democrats. The disparity is likely to grow because Romney and his Republican allies have more money in the bank for the fall.

Obama advisers say that, even if he is outspent on the airwaves, they are confident that his massive ground operation and get-out-the-vote efforts will tip the scales. Romney spent nearly all the money that he raised during his fight for the GOP nomination earlier this year, while Obama used that time to open offices and hire staff.

“Grass-roots giving is what’s powering this campaign,” said Obama campaign spokesman Adam Fetcher. “Our donor base is as diverse as the country is, and we are succeeding at reactivating longtime supporters and finding new ones.”

Nonetheless, the campaign frequently casts Obama as a financial underdog in desperate straits, a far cry from the optimistic messages of four years ago. One fundraising plea last week from Obama began, “My upcoming birthday next week could be the last one I celebrate as president of the United States . . . ”

At a July 23 fundraiser in Oakland, Calif., Obama told the crowd that “over the next four months, the other side will spend more money than we have ever seen on ads.”

“Now I’m asking for your help,” he said a few moments later. “Now I’m asking for your vote. I’m asking you to knock on doors and make phone calls and do all the things we did in 2008.”

Josh Hicks in Florida, Laura Vozzella in Virginia and Matt Zapotosky in Ohio contributed to this report.