If Mitt Romney is victorious in Illinois on Tuesday, it will be due in part to one of his key advantages as a presidential candidate: big-money fundraisers.

Romney has assembled a broad roster of Illinois donors who have helped him bring in millions in President Obama’s back yard, part of a network of hundreds of major fundraisers nationwide that has allowed him to easily outpace the rest of the Republican field.

That financial advantage has been clear in states such as Michigan, Ohio and, now, Illinois, where Romney and an allied super PAC have outspent challenger Rick Santorum on the airwaves by a ratio of 7 to 1.

In Illinois and around the nation, Romney has built a web of campaign bundlers and other top fundraisers that stands to rival Obama’s, with more than 500 supporters who have raised at least $10,000 for the campaign, according to a review of fundraising invitations, state finance committee lists and other reports.

The top ranks are dominated by hedge-fund managers, bank executives and other members of the financial industry, along with a significant number of energy executives, real estate developers and other business owners, the review shows. The numbers are imprecise because, unlike Obama, Romney has declined to release a list of his top bundlers, who host fundraising events and help collect checks for the campaign.

More than three dozen of Romney’s biggest fundraisers are in the financial hub of Chicago, where a cross section of the city’s business elite has rallied around the former private-equity manager as the best candidate to beat Obama in the fall.

Romney’s relative success in attracting moneyed backers in Chicago is particularly striking given Obama’s roots in the city and his overwhelming popularity in Illinois. At least a dozen of Romney’s top Illinois bundlers supported Obama in 2008, but have turned on the president over his handling of the economic recovery, Israel or other issues.

Goldman Sachs managing partner Muneer Satter and his wife, Kristen Hertel, donated more than $100,000 to Obama’s 2008 campaign and inaugural committee, for example, and also supported former Obama chief of staff Rahm Emanuel’s mayoral bid in Chicago. But Satter now serves as Romney’s finance chairman in Illinois, and the couple have given a combined $380,000 to a super PAC supporting the GOP candidate, records show.

Susan Crown, a philanthropist and member of the billionaire Crown family, has switched sides from supporting Obama in 2008 to backing Romney now, although some of her relatives remain Obama supporters. Crown said in an interview that she was prompted to support Romney after Obama spoke in favor of a return to 1967 borders with land swaps as part of a potential Israeli-Palestinian peace settlement.

“There are a lot of people here who are very disappointed in the president,” said Crown, who co-hosted a fundraiser for Romney last year with her husband. “Illinois is very important because it’s Barack Obama’s home state and it has a lot of independents. I’m an independent and I’m working as hard as I can for Mitt.”

Romney has raised about $75 million for his primary campaign, drawing heavily from donors giving the maximum of $2,500. But he has burned through nearly all of that money in pursuit of the nomination, putting him well behind Obama, who raised $45 million in February alone for his campaign and the Democratic National Committee.

The money race has forced Romney to sandwich regular fundraisers into his frenetic primary schedule, including a flurry of events last week in New York that netted an estimated $3 million. Romney has scheduled at least three fundraising events in Illinois during his visit to the state for Tuesday’s primaries.

Tyrone C. Fahner, a longtime GOP fundraiser and partner at Mayer Brown in Chicago, said that Romney’s fundraising prowess reflects the strong support that he has in the business community and helps explain his strong poll numbers among Republicans in Illinois.

“We’ve raised a lot of money for him,” Fahner said. “I think that’s part of why he’s doing well here. The money isn’t everything, but it does help get the message out.”

Interviews with major Romney donors in Illinois and other states suggest that the reasons for his popularity within the business class tend to be similar around the country. Many hedge-fund and private-equity managers, for example, oppose Obama’s efforts to eliminate tax loopholes that are favorable to the industry. Others argue that Obama has been reckless with the country’s finances and has not done enough to turn around the economy, while still others fault his energy policies.

Romney said at the University of Chicago on Monday that Obama’s policies have been “an assault on our economic freedom” that have slowed the recovery.

“What I hear from others is that President Obama has created a burdensome environment for free enterprise and somewhat of an attack on capitalism and free markets,” said Damon Watkins, a Romney fundraiser and vice president at Melaleuca, an Idaho-based supplement firm that has given $1 million to Restore Our Future, a Romney super PAC. “Governor Romney brings a straightforward, pro-business attitude that really seems to resonate.”

Reeve B. Waud, founder of Waud Capital Partners, a Chicago-based private equity firm, said major donors to Romney are motivated primarily by economic concerns and the belief that his business experience would be an advantage in the White House.

“I would say he’s connecting well with a broad range of business people,” said Waud, who held a fundraiser for Romney at his North Shore home last year. “The perception is that he’s stronger among the financial community, but I would say it’s a much broader base than that.”

Nonetheless, the finance sector is crucial to Romney’s fundraising efforts, which include numerous bundlers with ties to Goldman Sachs, Bain Capital and other major investment firms. Out of more than 500 donors identified as fundraisers, at least 150 work in the finance and banking industry, records show.

Romney supporter Peter Fitzgerald, a former U.S. senator from Illinois who now heads Chain Bridge Bank in McLean, said the candidate’s appeal to the business community is a key part of his success in amassing money and delegates. But he said Romney also must appeal to other parts of the GOP coalition, a test that he expects Romney to pass in Illinois.

“Romney has the best fundraising operation and he is the candidate with the most support in the business community,” Fitzgerald said. “But that definitely isn’t enough.”