RICHARD LARDNERWASHINGTON (AP) â¿¿ Mitt Romney's presidential fundraising operation dwarfs those of his Republican rivals, with more than $75 million already in donations. It also operates mysteriously at times, withholding the names of Romney's major fundraisers who have helped amass much of its money. Now, a review by The Associated Press of campaign records and other records provides clues to the vast national network of business leaders bringing in millions to put Romney in the Oval Office. The AP's review identified dozens of people who fit the profile of top Romney fundraisers, known as "bundlers" for their ability to sweep up donations from wealthy acquaintances and steer them to campaigns. At least seven are the mega-rich donors who each gave gifts of at least $1 million to an allied pro-Romney political committee. Dozens more were listed on invitations for fundraising events, assigned to mine their business and personal networks for maximum campaign contributions. The AP identified likely Romney bundlers through interviews, finance records, event invitations and other publicity about campaign events. Romney's campaign will not identify his major fundraisers â¿¿ unlike President Barack Obama's organization, which in January disclosed both bundlers' identifies and their fundraising thresholds. Federal law does not require the Romney campaign to divulge the names, but both GOP and Democratic presidential candidates in recent years routinely provided the identities and money ranges of their top fundraisers. The lack of transparency by the Romney campaign about its top bundlers prevents voters from knowing who wields influence inside the GOP frontrunner's campaign and how their interests might benefit if he is elected. Romney is in California this week for at least five private fundraisers â¿¿ typically off-limits to media coverage. Even in the era of "super" political committees, which can pull in millions of dollars in unlimited and effectively anonymous contributions to support candidates, bundlers are their own campaign forces. Unlike super PACs, which under federal law are not supposed to coordinate with candidates, bundlers raise large amounts that are deposited directly into a campaign's bank account â¿¿ money that can be spent to pay for salaries, get-out-the-vote efforts and advertising.