CHICAGO (AP) — With Republicans locked in a contentious and expensive primary, President Barack Obama has spent a small fortune in recent months to build and maintain a campaign operation that is larger, more diverse and more focused on November's general election than any of his opponents' organizations.

Republican contenders like Mitt Romney and Rick Santorum have surely been watching their expenses during their primary elections: millions here for ad spending, millions there for travel, rallies and consulting fees. What's left keeps the lights on, the phones ringing and the staff paid.

But Obama, who faces no serious challenger for the Democratic nomination, has sunk his cash into an expansive brick-and-mortar operation with offices in nearly every state. His campaign has spent more than $135 million on operations through February, according to an Associated Press analysis of Federal Election Commission records. That's about $3 million more than all his GOP challengers combined.

Republicans bristle over reports that Obama's paid staff exceeds 500, many of whom work in the campaign's Chicago headquarters.

"I think the campaign is single-handedly trying to lower the unemployment rate by hiring field staff," Romney political director Rich Beeson said. "When they point to the fact about how many people they've got hired and how many offices they've got, they're just trying to distract people from the reality of (how) they're going to have a heck of a time finding people to get out and vote for him."

A review of Obama's balance sheets reveals a small army of paid staffers trying to help the Democratic president win a second term. Campaign filings list more than 330 paid staffers in Chicago and 200 more spread across the country — payroll costs that exceed $6.3 million during the last two months alone.

The numbers also suggest signs of stress.

Obama's team has spent more than $7 million since January on fundraising-related expenses like postage, printing and telemarketing, in some cases to contact the same kind of low-dollar donors who supported Obama four years ago. The campaign also spent millions more on expenses like online advertising and consulting, which in some cases can be tied to fundraising.

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With offices in nearly every state, the campaign also faces rising overhead. Through the first two months of the year, Obama spent approximately $1.1 million on computer equipment, $435,000 in rent and utilities, $305,000 on telephones, and $19,000 on office supplies, federal filings show.

"We're building the largest grass-roots campaign in history," campaign spokesman Ben LaBolt said. "You can see it here, but it's really happening in the states."

The core of Obama's operation is packed into the sixth floor of Chicago's Prudential building, where 300 staffers sit side by side at long rows of tables, working from laptops and cellphones. Colorful college pennants hang from the ceiling and often represent key swing states: the University of North Carolina, Ohio State and the University of Michigan. Need a designer T-shirt or bumper sticker? A room managed by two staffers houses a swelling collection of campaign memorabilia for sale.

In one corner, more than a dozen workers field questions from journalists scattered across the country. Elsewhere, others coordinate media appearances for Obama's high-profile supporters. Other staffers focus on fundraising, voter identification, social media and campaign-finance reporting.

Beyond the Windy City, Obama's campaign said it's already opened five field offices in Arizona, a state it expects to be increasingly competitive in the fall. The campaign is also taking advantage of party resources there, relying in part on the state's Democratic Party for staffing, phones and computer equipment, records show.

Obama's operation had $84.7 million in cash-on-hand by Feb. 29. But the Romney campaign — which is hardly hurting for cash after raising about $74.8 million — says it's not impressed.

Romney's Boston-based campaign is a fraction of the size of Obama's, although its organization dwarfs its Republican competitors. With around 100 paid staffers, Romney has spent more than $180,000 in rent and utilities since early January in at least 16 states, records show, from Massachusetts to Utah. Santorum, whose national headquarters was technically a post office box until this month, spent just $19,000 since the beginning of the year on rent-related expenses.

Romney's ability to compete against Obama's growing organization has become a selling point on the trail.

"As Republican primary voters in Illinois, we have an opportunity to look at the field of candidates, and look at them and say, 'Who can go toe-to-toe not just with Barack Obama, but the Chicago machine that is his operation?'" Rep. Aaron Schock, a top Romney supporter in Illinois, said recently. "We cannot afford to nominate someone on our ticket who cannot withstand the barrage, who does not have the organizational strength and fortitude to go toe-to-toe with Barack Obama."

Obama's massive campaign is part of what drives supporters to give money to "super" political action committees popping up this election cycle. Major super PACs like American Crossroads, which supports Republicans, and Restore Our Future, which supports Romney, have tens of millions of dollars apiece to help, thanks to federal court rulings that have stripped campaign-finance rules to allow unlimited — and, at times, effectively anonymous — donations from billionaires, corporations and labor unions.

"As a general rule, the big institutional money goes inordinately to incumbents regardless of party, while outside money plays the role of balancing out the party in power or stopping a specific initiative," Crossroads spokesman Jonathan Collegio said. GOP-leaning super PACs have spent more than $50 million on TV ads this election; Crossroads is largely waiting to spend its money until the general election.

Whatever the case, the Democrats' current financial advantage is something Obama's campaign isn't taking for granted. Obama changed course last month in his criticism of super PACs and began encouraging big-money supporters to give to Priorities USA Action, a group working in his favor.

The Republican Party has opened offices in three states this month and plans to expand to four others in April.

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Gillum reported from Washington.

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Follow Jack Gillum at http://twitter.com/jackgillum and Steve Peoples at http://twitter.com/sppeoples