THE DIFFERENCE between President Obama and presumptive Republican presidential nominee Mitt Romney when it comes to fundraising is not only that Mr. Romney managed to outraise the president last month. A more troubling difference is that Mr. Romney provided almost no information about the key “bundlers” who helped his campaign vacuum up such huge sums.

This omission distinguishes the former Massachusetts governor not only from his Democratic counterpart but from his two Republican predecessors. Both President George W. Bush, during his two campaigns, and Arizona Sen. John McCain, during his 2008 presidential race, released lists of their key fundraisers and, at least within general parameters, some indication of their hauls. But Mr. Romney’s campaign has repeatedly dismissed suggestions that he follow suit. The campaign has said that it has complied with campaign finance laws, which do not mandate such information except in the case of registered federal lobbyists.

This argument is correct but unconvincing. Bundlers play an essential role in presidential campaigns — Mr. Romney’s in particular. The latest figures show that just 15 percent of the Romney campaign’s May total came in donations of $200 or less, compared with 40 percent of contributions to the Obama campaign. Such hauls of maximum-dollar checks do not happen without the assiduous behind-the-scenes work of well-connected bundlers.

The snippets of disclosure of lobbyist-bundlers illustrate that point: An analysis of Federal Election Commission filings by the Public Campaign Action Fund found that 25 lobbyists had bundled more than $3 million for the Romney campaign through April, including a Barclays Capital executive, Patrick Durkin, who was near the $1 million mark ($927,160.)

A Romney campaign pamphlet prepared for donors details the special perks lavished on those who haul in the biggest bundles: “Stars,” who raise $250,000, and ”Stripes,” who bring in $500,000 or more. “Stripes” receive everything from a “dedicated Romney Victory headquarters staff member” to access to debates to “signature membership pieces and apparel.” Indeed, the campaign promised to publish the information it declines to reveal to the public by listing bundlers’ names in a ”Stars and Stripes Commemorative Book.” The Post reported Friday that the Romney campaign’s “top 100 bundlers — ‘Team 100’ — are heading to a private retreat in Utah on June 22-24 to plot out how to replicate the May success in the months ahead.”

Who are these men and women to whom the campaign is so deeply indebted? Mr. Romney has yet to answer — indeed, he has yet to be directly asked — why his campaign will not live up to the standard set by Mr. Bush and Mr. McCain. Why would he hide his roster of key supporters?