By law, Romney and other candidates must reveal the names of bundlers only when they are lobbyists.
However, bundlers -- those who are themselves huge donors and press friends and associates also to contribute -- have become more and more influential after campaign finance reform in 2002, and now account for hundreds of millions of dollars that help underwrite national campaigns, The Boston Globe reported Monday.
"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," Richard Briffault, a Columbia Law School professor, told the Globe.
Critics say the Romney campaign's failure to reveal the names of his bundlers signals that Romney is out of sync with previous Republican presidential candidates, opponent President Barack Obama, as well as his own practice when he sought the Republican presidential nod in 2008.
Obama revealed the names of his bundlers in 2008 and is doing so again this election cycle, his campaign said.
The Romney campaign won't say why it has resisted pressure to reveal names, the Globe said.
"We disclose all of the information about our donors as required by law, and anyone who is interested can review it publicly," Romney campaign spokeswoman Andrea Saul said.