Some of Mitt Romney’s largest Massachusetts financial backers are well known. His former partners at Bain Capital have given more than $3.3 million. New Balance chairman Jim Davis gave $500,000, and retired Reebok International chief Paul Fireman has contributed $250,000.

Their support is public because they have given money to a Romney super PAC, which is required under federal law to disclose the gifts it receives. But there is a whole other swath of influential Romney supporters whose names voters can only guess at, because the campaign refuses to reveal them.

They are a category known as “bundlers,’’ people who write big checks themselves and press friends and associates to donate. They became increasingly influential after campaign finance reform in 2002, and now account for hundreds of millions of dollars that fuel national candidacies.


“In some ways bundlers are more important, or at least as important, as the donors themselves, because someone who is able to bundle half a million dollars together has influence,’’ said Richard Briffault, a professor at Columbia Law School.

By law, Romney and other candidates have to reveal only the names of bundlers who are lobbyists. But, Briffault said, “It goes toward the whole notion of transparency in the campaign finance system. Obviously, the money matters.’’

If their candidates win, bundlers are often richly rewarded with ambassadorships and other administration posts, or personal access like overnight stays in the White House. According to the Center for Public Integrity, which tracks campaign finance issues, about 80 percent of Barack Obama’s and George W. Bush’s bundlers who raised $500,000 or more went on to win appointments.

Critics say that by failing to disclose the names of his bundlers, Romney is out of step with past Republican presidentidal candidates — and his own practice in 2008 — as well as that of his Democratic opponent, President Obama. Obama revealed his bundlers’ names in 2008 and is doing so again this election year.

The Romney campaign won’t say why it has declined to reveal names, despite pressure to do so from advocacy groups and the Obama camp.


Said Andrea Saul, a spokeswoman for the Romney campaign: “We disclose all of the information about our donors as required by law, and anyone who is interested can review it publicly.’’

Bundlers are often prominent supporters in cities across the country who hold house parties at which the candidate appears and attendees dig deeply into their pockets for the chance to rub elbows. The goal is to get lots of friends to give the $2,500 maximum donation to the candidate. Donors also can give up to $30,800 per year to a political party.

Super PAC donations have no limits and cannot be directly connected to the campaign.

President Obama’s bundlers in Boston are a who’s who of successful business people, from hedge fund executive Scott Nathan and retired advertising mogul Jack Connors to Bain Capital partner Jonathan Lavine and public relations veteran Larry Rasky. Each has raised more than $500,000 for Obama, according to the campaign’s disclosure and figures put together by the Center for Responsive Politics, a Washington group that tracks money in campaigns.

Clues about Romney’s bundlers, by contrast, must be gleaned from event invitations, eyewitness accounts, and news reports. For instance, Reebok’s Fireman and his wife, Phyllis, are hosting a fund-raiser at their home in Chestnut Hill on Sept. 28. As hosts, the Firemans have committed to raise $100,000, while “VIPs’’ will bundle $50,000, and $10,000 will fetch a photo with the candidate. Last month, billionaire Bill Koch threw a $50,000-a-head fund-raiser for Romney at his home in Osterville; others hosted events on Nantucket and Martha’s Vineyard.


Putnam Investments chief executive Robert Reynolds attended the Nantucket event but said he is not a bundler, just an individual supporting Romney, whom he considers a friend. He also has given $100,000 to the Romney super PAC. He said he is not troubled by the Romney camp’s lack of disclosure on bundlers.

“To me, bundlers is a very definitional word. What is a bundler? I think it’s hard to define,’’ Reynolds said. “If Obama’s having a reception here — he’s had a couple hosted here — and the person invited 20 people, is he a bundler?’’

Andy McLane, a senior adviser at TA Associates, a Boston private equity firm, who has given $100,000 to the Romney super PAC, said it does not matter if the campaign reveals its bundlers to the public. Republican insiders know who the big backers are, he said.

“In my view, who does the asking doesn’t matter much. What counts is who is writing the big checks,’’ McLane said.

To be sure, some of the biggest checks are going to the super PACS, which have no limits. The Romney-supporting group Restore Our Future has garnered $10 million from Las Vegas casino owner Sheldon Adelson, and donations from numerous locals like McLane, Fireman, and Reynolds.

The group Priorities USA Action

, which supports Obama’s reelection bid, counts among its top donors film producer Jeffrey Katzenberg, at $2 million. Locally, it has drawn $250,000 from Weston entrepreneur Paul Egerman, founder of the technology company eScription, and $100,000 from Maynard biotech executive Reinier Beeuwkes.

Some super PAC donors are bundlers, too, but not always. Republican nominee John McCain disclosed his bundlers in 2008, as did Bush in the prior two elections. In fact, the Bush campaign raised the profile of bundlers by giving them names based on their level of giving: Pioneers raised $100,000 or more; Rangers raised twice that. Super Rangers raised even more.

Obama’s bundlers so far have provided $143 million in total to the campaign and to the Democratic National Committee, according to the Center for Responsive Politics. As Romney discloses only the lobbyists who are bundlers, his number appears much lower, at $5.3 million.

But the real number is almost certainly much closer to Obama’s, according to campaign finance specialists. The Romney campaign brought in $111.6 million in August’s rush of political fund-raising, edged out slightly by Obama’s $114 million. Much of that came from individual giving, but bundlers also played a role.

The rewards for bundlers come with both parties. Locally, philanthropist Elaine Schuster, a big Obama supporter, was named a delegate to the United Nations General Assembly. In the Bush years, EMC Corp. founder Richard Egan, a major fund-raiser, was named ambassador to Ireland.

The Obama campaign has tried to pressure Romney to release the names of his bundlers, to no avail. Obama campaign spokesman Michael Czin said, “President Obama has brought unprecedented openness and transparency to the campaign trail and the White House while Mitt Romney is running one of the most secretive presidential campaigns in memory.’’

Connors, the Boston power broker and retired advertising executive who is an Obama bundler, called Romney’s refusal to release his major fund-raisers disappointing, adding, “just like I think it’s disappointing he doesn’t release his taxes.’’

Asked whether Romney was out of step with his own party’s precedent on disclosure, a spokesman for the Republican National Committee said it’s a decision that’s up to each candidate. Romney complies with the law, reiterated RNC spokesman Sean Spicer, and “if candidates choose to do something beyond that, that’s their decision.’’