Former Massachusetts Gov. Mitt Romney (R) is refusing to release the names of most of the bundlers raising money for his presidential campaign, with the exception of the names of registered lobbyists as required by federal law.

One bundler, Ogilvy Government Relations chairman Wayne Berman, is a registered lobbyist for Fannie Mae, the largest U.S. mortgage finance provider and a frequent target of Romney's criticism. Berman gathered at least $279,075 for the candidate in 2011, according to The Associate Press.

Romney's likely Democratic opponent, President Barack Obama, released the names of all his bundlers last month and refuses to take money from registered lobbyists.

Since 2000, all major presidential candidates have identified the names of their fundraising bundlers. Romney, along with former House Speaker Newt Gingrich and former Pennsylvania Sen. Rick Santorum, is bucking the trend by keeping the names a secret.

"The lack of disclosure prevents voters from knowing who wields influence within a presidential campaign," the AP noted. "Keeping their identities secret could end up stinging Romney — like the mishandled release of his income tax returns — if voters conclude he is withholding politically damaging information."

Romney could also get hit with charges of hypocrisy after he accused Gingrich of "cashing in" on the mortgage crisis because of his ties to mortgage giant Freddie Mac.